Psychologically speaking... Why do some women avoid having sex?

[Nathan L]

I'm sure everyone has heard stories of girls being afraid to have sex for the first time. Also some girls, who have had sex, all of the sudden say that that don't want to have anymore sex until they are married. It's not like at that point that they are saving their virginity for marriage. I am just curious as to why girls wish to take away something so vital to a relationship's vitality? And why are girls afraid to take that first step with a guy? Even if both parties are "in love" and both agree it would feel great... And to throw out another curve ball, what if both parties are protected, STD free, and horny? To a guy like me, that means the perfect opportunity to have sex. Any answers would be helpful from both males and females