sometimes, it's not a matter of being unfaithful at all...
i'm an australian-born asian and i think half the people here have had multiple partners at some point during high-school or uni.
my first caucasian girlfriend had probably a lifetime of sex before she left highschool. prior to that, i'd been with two asian girls, who were both virgins. in any case she had sex with three guys on a hotel balcony in grade ten, was screwing her english teacher in grade 11, blew a friend at the movies, so on and so forth. it hurt me to hear all of this at first but it really opened my eyes to what goes on in the world. six months into our relationship, i went to a music festival with her and her best friend and her best friend's boyfriend. we shared a huge tent and basically it was everyone getting stoned and fucking each other non-stop for three days. so yes, i had sex with my GF's best friend there and after she left her guy, she came over to our place a handful of times to join us. after that, i felt a lot more accepting of girls with a big apetite for sex, it's just what some girls do these days.
in any case, she had been on "the pill" since she was 15 and perhaps this made her ridiculously horny and dripping wet all the time, if she wasn't naturally already. one of the first things i noticed that was different to my previous relationships was that i was literally being r***ing when i really needed to do other things - it's sometimes scary when you can see it in her eyes. i once asked her how much she'd masturbate if she were alone, and she said at least once a day or more if possible. we once had sex seven times one sunday, and that's the most that either of us had ever managed since neither of us had a place of our own before uni. so at least that's one more landmark to add to her sexual history. oh that and i had my first premature ejaculation... it was my first time having sex with a blonde girl, and also without a condom, i didn't realize how hot and wet it was and after a few thrusts i had one of the most memorable orgasms inside her
we broke up because she wanted to change to a different course/uni and i don't think either of us could manage the long distance thing. i was like an addict in withdrawal when she left, but luckily we both found someone new about two months later. in any case none of the girls I've been with since then has had her kind of libido.