As long as people are making up theories, here's mine:
I think the boy may have said something "kinky" in the heat of sex. Something that's offensive in any other context than hot, inter-racial action. Only she didn't take it that way.
Combined with the fact that her own college has a two-bit name, she is stripping/getting boned for and by a younger guy who had his dad pay for school, while she is paying her own way and raising a baby... Age 27 is a bit delayed for all of that (especially for women) in contrast to the boy who is going to be 25 and maybe own a Corvette by then.
I believe all of these factors suddenly came to light for her that night, and she just couldn't take it.
Except that, I'm willing to bet big coin, there was no sex, of any kind.