Why don't girls ever initiate sex? Are girls afraid they will seem slutty or do they think it's embarrasing to initiate sex? Or do girls not like sex at all?
First of all, many women do initiate sex! The problem is that while sex is viewed as a positive thing for men, it is often viewed as negative for women. Women are pressured to have sex to fit in and make guys happy, but if they have too much sex and not in the "proper" context (a monogamous relationship while they're in love) then they can be branded a ****. By letting a man make the first move, they're doing what they've been taught, which is to be safe and let men decide when they should have sex. In this fashion, sex is reactive rather than proactive for some girls. After all, when you watch movies, the only kind of woman that initiates sex is that dangerous, slutty porn star type, right? Additionally, our society is very porn-oriented, so young people are likely to think that sex is about a man's orgasm. As a result, a young girl's first experiences with sex will probably be unsatisfying and not result in an orgasm for her. The conclusion is that most women have a problematic relationship with sex and are pulled between liking it and doing it for other people. The 30s is considered the sexual peak for women not exactly because they have more testosterone then, but because by that time they've had a few good lovers that knew what they were doing and have learned to enjoy their sexuality on their own terms. Finally, not every woman has a problematic relationship with sex. Things are changing, and more and more we're able to do it for ourselves at younger ages. Believe me, women that know how to have sex confidently, for their own pleasure, often can't get enough, and have no qualms about bugging guys for it! Just ask my poor husband.
I really don't think you can make that kind of blanket statement about girls not initiating sex.
In a relationship, it's very individual and many women certainly do initiate sex. Are you asking because in your relationship, your partner never initiates? Or maybe that none of your partners have? It's very personal really, and maybe that's just the type of partner you have.
Young, inexpereienced, and unsatisfied women don't innitiate physical intimacy. Women who know and enjoy their bodies has no problem innitiaiting. Early on women don't have confidence that they acquire with age and experience which often hinders their ability to "come on" the their partner.