The woman you were talking to was an idiot. For proof look no farther than identical twin studies that show that while sexuality has a large genetic component, there is an environmental component as well. Furthermore people can be confused over their sexuality - either way - for a very long time. And other people don't have an easily classified sexuality.

To give an interesting example, how would you classify a male who is mentally a woman (had not yet had the operation) and into BDSM with both genders? (For the record, I don't do hypothetical examples ...I know enough interesting people that I don't have to.)

In general, if you have a simple model of how people work, it is almost certainly wrong. And if it is both simple and in accord with your personal politics, the likelihood of it being wrong goes way up.