The penis goes into the vagina very easily, although there are many stages so it goes in "right." When the penis goes into the vagina it may go in far enough that - if the female is a virgin - it may tear the small layer of tissue that partially covers the vaginal opening, known as the hymen. When the female is not a virgin, depending on the elasticity of the hymen, the penis should slide in without any difficulties.
The vagina is actually the name of the canal in the woman's body, a natural orifice like the mouth or anus. During sexual intercourse, the male's penis is inserted into the vagina. If the penis even comes in close contact with the area near the opening of the vagina, whether the male ejaculates or not, it can result in pregnancy (the woman can conceive a baby).
The ejaculated fluid is called semen, and contains the male sex cells, or sperm. If the sperm cells enter the vagina, they will attempt to fertilize an egg cell from the ovaries, which will become a fetus and eventually a baby.
Note: To avoid injury or discomfort, be sure that the vagina is lubricated before intercourse. Normally body contact is sufficient to begin this process. If natural fluids do not provide a comfortable insertion, there are artificial lubricants
Using a Condom:
Condoms are usually lubricated, maybe not enough, but it helps a lot and they're smoother. This may make the process easier, thanks to the added lubrication.
The most important thing is to first be sure that the female is receptive and has given permission for the penis to be inserted into her vagina. That is step number one!
1. Begin by ensuring the vagina is properly lubricated, either through oral or manual stimulation or a store bought lubricant.
2. Inserting the penis into a dry vagina may cause the female a painful and unpleasant experience.
3. Proceed slowly by using the head of the penis to feel for the opening of the vagina. Do not stab at it. Be gentle.
4. Place the head of the penis into the vagina and slowly lean in to finish the insertion of the penis.
The penis is now inserted and sexual intercourse can proceed.
The best way to approach this is to make sure the girl is adequately aroused. You do this through foreplay (sexual touching and caressing before intercourse.)
You can also use vaginal lubricants
Factors that may prevent insertion would include; the penis is not erect enough, the vagina is not wet enough or lubricated, the vaginal opening is too small for insertion
When a man is sexually aroused, his penis becomes longer and stiffer. This is called an erection. Where a man's penis usually is soft and hangs downwards, when he has an erection it is hard and points forwards and slightly upwards.
When a woman is sexually aroused, some lubricating fluids are produced by the Bartholin's glands which are located close to the opening of her vagina which can make it more slippery inside.
So the combination of the man's and the woman's sexual arousal makes it very easy for the man to put his penis inside the woman's vagina or for the woman to take the man's penis into her vagina.