It's sexual dynamics. With all the precautions, there is always the chance of getting pregnant - so, the guy better be worth it. Then, it's the sex itself. He drives his penis into you. Hopefully, he knows how to use it, cause when he's done, who knows?
If a lady is just straightforward and aggressive she can have as much lesbian sex as she can handle. Women are always thinking about other women, it's just a fact.