Well, I have been out of the dating scene for some time, but is seems to me if anyone would have casual sex that quickly they could be carrying some STDs including AIDS.
All the casual sex in this movie and STDs aren't even considered. Hmmmm
I think that the proper term to use now is an STI. And they don't consider AIDS to be an STI or STD anymore, because it's transmitted by blood to blood contact, and rarely through (heterosexual) sexual encounters.
I was thinking that too. Especially because she had her tubes tied when she was 17, and I assume never used protection after that. But, it's not an after-school special, we don't need to add another story line where you see Samual L. Jackson buying a special shampoo and learning how to treat his newly acquired STI.
AIDS are indeed spread by heterosexual encounters as well as in homosexual encounters, specially nowadays when a lot of people are bisexual and have casual sex outside their relationships.