I'm not following your argument. You say that if a woman gets pregnant, then it will be a big change in her life because of the investment in the child. Fair enough.
But then you go into virginity and it's importance before marriage. Ok, but marriage, pregnancy and viginity are three separate things.
Let's start with virginity. You seem to be equating virginity with a hymen. Which is wrong because A) It is completely possible for a hymen to remain intact after sex, albiet rare. B) It is possible for a woman to be born without a hymen, or break it before her first sexual experience, even from non-sexual causes. C) Hymens can be surgically reconstructed.
Now let's talk about sex. It's possible for both virgins and non virgins to have sex. There is no special distinction between the two given the above other than level of experience/naivety. Also, sex does not equal pregnancy. It's completely possible to have sex and not get pregnant, especially if contraception is used. Hell with today's technology it is possible to get pregnant without sex.
Now pregnancy. Pregnancy generally results from sex, it is true. But relationships don't come to a halt because of pregnancy. Many women continue to have sex well into pregnancy. It is completely possible for a woman to cheat on her partner while pregnant. Does the fact that she can't get pregnant from this act really make it less bad?
Which brings up marriage. People of any combination of virginity statuses can get married. That doesn't mean that some marriages are inherently less commited. Sex and marriage aren't synonymous. There are unmarried couples that bang each other like a bunch of rabbits and married couples who have never even been physical, although this latter category is understandably rare. Finally their are unmarried couples who have an entire brood and married couples that are deliberately childfree.
Your entire argument also ignores the existence of stepfamilies, gay couples and infertility.
Sorry about that. I got stuck in rant mode again.