Facts About Sex, Anatomy, Pleasure and More

“Intromission” is the technical term sex therapists and doctors use for the penis entering the vagina (or anus for that matter) during intercourse.

Another word people often use is penetration.


Intromission or penetration is only a small part of a healthy sexual relationship between two people. Unfortunately, it is often portrayed in films as the only type of sex people have, as the only “real” sexual act.

However, being sexual can entail so much more than just penetration. If you want to find out more about other kinds of sex please read our page on foreplay.

Although there are many other ways to be sexual, penetration will always be special to people as it is the “natural” way to conceive a child. (Of course, ejaculating semen onto the vaginal opening without penetration also has the potential to result in pregnancy, but the likelihood will be much smaller. )

Additionally, intromission of the penis into the vagina is often the only way to have sex acceptable to some religions. On the down side, penetration is often not very stimulating to women, who may need more clitoral stimulation.

 How intromission works 

For intromission to work one needs three things: an erect or partly erect penis, lubrication and a relaxed vagina.

 The penis needs to be erect for intromission, at least to some extent, to give it the necessary stiffness to allow for penetration. This page – erections – will show you how the male body works.

If your penis feels hard to you when you squeeze it between two fingers, you’ll be able to get it in your partner’s vagina. Another way of checking is to hold your penis straight at the base when you lie on your back. If it points upward it will be hard enough for penetration, even if it isn’t completely rigid.

Most men experience some difficulty with their erection at some point in their life, especially during periods of stress or tiredness. However, you can still have a very good time even if your penis is not rock solid.

If you still want to go for penetration you or your partner could use a hand to help your penis inside. You may feel your penis stiffening up once you are inside her vagina.

If a man is experiencing frequent problems with erections, this is called erectile dysfunction. It is a very complex issue often involving physical problems, medical issues, emotional challenges and relationship issues.

Would you stop at the point  were sex becomes possible?
Would you stop at the point were sex becomes possible?

To read more about erectile dysfunction go to our sex problems page – or you could read Bernie Zilbergeld’s book, The New Male Sexuality, published by Bantam Press in 1999.

For penetration to be fun for a woman she needs plenty of lubrication.

The female body naturally produces some lubrication in the form of vaginal fluids. However, the amount of natural lube produced by a woman varies a lot – it depends on things such as her age, her level of stress, the point she’s at in her menstrual cycle, whether she’s had children, and her level of arousal.

Some women have plenty of vaginal fluids; others have almost none.

Lubrication is very important as the tissues around a woman’s vulva and vagina are very delicate and can get sore from friction very easily if they’re dry.

This may result in the woman experiencing penetration as uncomfortable or even painful, which will then lower her arousal resulting in even less natural lubrication. Most women would say the more lubrication, the more fun!

Lubrication can be provided in all sorts of ways and does not have to come from a woman’s vaginal fluids alone. Some couples use oral sex as a method of supplying lubrication (that’s the partner’s saliva!)

Another option is to use oils such as scented oils or massage oils. If you use oils, pick something that is gentle on the skin and membranes such as coconut oil or almond oil.

Do not use pure essential oils without diluting them as they will irritate the skin.

Additionally, do not use oils or oil based products if you use condoms for contraception

The oil will render the condom useless. These are oil in water mixtures with a much higher water content than oil based products. You can buy them in pharmacies, sex shops or online.

Another option for lubrication is a water based product such as KY Jelly or  aqueous cream. Water based lubricants are OK to be used with condoms.

They may feel more natural to a woman as they are similar in consistency to her own body fluids. If the woman experiences any irritation of her vaginal lining during or after sex, you’ll need to change the lubricant you are using.

Finally, for penetration to be possible a couple also need a vagina with relaxed muscles. The vagina is a powerful muscular organ, which is folded and collapsed when at rest.

The vagina is very flexible and can easily stretch to accommodate any size of penis when it’s sexually active. The outermost part is made up on the inside by muscles called the pubococcygeal muscles (or PC muscles for short).

These make up part of the wall of the vagina and are surrounded and supported by other muscles in the pelvic floor and perineal region (around the anus). 

A woman can use these muscles consciously. She contracts them when she stops herself from peeing or when she tightens up her vagina around a penis. The PC muscles are very important. If they are well-toned and flexible they contribute to good sexual health and increase sexual pleasure. They can be trained with Kegel exercises.

 However, some women are not very aware of their own sexual anatomy or the muscles that surround their vagina.

If a woman’s also a bit fearful of penetration or worried about real or imagined pain resulting from penetration, she may develop a condition called vaginismus. 

Video – Vaginismus

Vaginismus describes an unconscious response in which a woman’s vaginal muscles contract in spasms which close the vaginal entrance. The spasms are beyond a woman’s immediate control. It is as if her body contracts to protect her from potential pain, similar to automatically pulling back one’s hand when touching a hot surface.

The muscle spasms can be so strong that they actually feel painful.

Vaginismus is a “learned” response – i.e. it develops over time in response to circumstances – and can therefore be unlearned. To do this, a woman needs to learn more about her sexual anatomy as well as trying some behavioral training exercises. Sensate Focus may help to develop a sense of relaxation with physical contact in general.  

When a woman experiences vaginismus, penetration may not be possible at all, or may be painful to her. It is also possible that she is not aware of the contraction of her vaginal muscles: all the couple know is that her partner is simply not able to penetrate her.

Some couples do not know what is happening at this point. Therefore, we are attaching some clear pictures of the process of penetration or intromission below, so that you can see what it looks like.

Penetration should be possible easily, with it being potentially a bit more tricky during first time sex. The vagina cannot be built too small!

As a woman you should be able to easily introduce one to three fingers into your vagina with lubrication. If this is difficult for you, please read the page on vaginismus or consult a specialist doctor.

[Note by co-author Rod: I agree in theory with these comments about vaginal size by Anna but I have heard from many Western men in relationships with Asian women, and they mostly tell the same story: Asian women have smaller vaginas than Western women. Moreover, from what these women say, it seems that Asian men have somewhat smaller erections than Western men.

Video – Penis Size

This limited anecdotal evidence leads me to think there may be some size differences between races, though why this should be so remains a mystery. Of course I should add that I haven’t heard from the Western men and their Asian partners who are matched well in size.

On the other hand I have had pleas for help from Western women and Western men who were in a relationship where the man had a large penis. There can, it seems to me, be size challenges for all men and women, though it’s true they are quite rare. Generally, a penis and a vagina will fit together well.]

Video – penis size

Pictures of intromission or penetration

The following pictures are to give you a clear idea of what happens during intromission. They are all shot with the couple being in the man-on-top position.

Our apologies for having only images of a man with a rather big penis, it’s not that easy to find other men to come forward! He is endowed beyond average size, which is hopefully reassuring to know if you’re a woman who struggles with the idea of penetration. 


The picture shows the start of intromission. The glans of the penis has already entered the vagina, but the shaft has not.

To make penetration easier the women could reach down with her hand and guide the penis to the right position just below her pubic bone. She could also hold on to the penis to get better control of penetration and feel safer with the process.

As the man gently enters, his penis vanishes completely in the vagina so that his pubic bone and hers touch.

This should be easily possible for the woman without any pain or discomfort. The only potential  discomfort which may arise is if the tip of the penis touches the woman’s cervix deep on the inside.

The cervix is the bottom part of the uterus and forms part of the inside end of the vagina.

The muscles around the uterus contract when a woman becomes sexually aroused, and this lifts the uterus and cervix out of the way of the penis.

However, in some sexual positions or when she isn’t very aroused, the penis may touch a woman’s cervix briefly. This should be the only slight discomfort for women during penetration. It can easily fixed by her moving her body slightly to change the angle of his penis inside her.

Any other discomfort for the woman during penetration needs to be looked into. It could be due to a medical condition such as an infection, or a condition called dyspareunia, where penetration is painful, or vaginismus.

If you do experience any ongoing problems, please consult a specialist: a sympathetic medical doctor or a sex therapist (also called a psychosexual psychotherapist).

The Greatest Show Of Sexual Positions You Will Ever Find!