I'm vaguely familiar with what you're talking about.

I remember a few years ago there was a study that came out about how different people reacted to sexual stimuli. If I remember correctly, essentially what they found is that men were only stimulated by what they were attracted to i.e. straight men were stimulated by things involving women, gay men were stimulated by things involving men. They also found that (most) women, regardless of their orientation, were stimulated by (for the most part) anything sexual (keep in mind that this included both straight women and lesbians).

I wonder if this could be the study that your teacher was refering to. If it was then, I think it would be a bit of a leap to come to the conclusion that lesbians don't exist.

First, if you think of it from a historical context, women were once treated as lesser than men and it was socially acceptable to take advantage of them. So, I think it's worth considering that perhaps women evolved to be more easily stimulated in response to being raped.

Second, as I said, the study concluded that women as a whole were stimulated by sexual stimuli regardless of the content of the stimuli/one's orientation. If we were to jump to the conclusion that "sexual orientation in women is fluid" and/or "lesbians don't exist" then that would mean straight women would also have to be sexually fluid and non-existent, which we know to be factually incorrect.

The only real way to understand exactly how all of this works would be for more studies to be done but, as someone else said, more studies have been done on gay men than lesbians.

Tl;dr: To answer your question, if you really want to argue with people, ask them "there is no 'gay gene' and there is no 'straight gene'. DNA is not able to predict someone's sexuality." Fight "science" with science.

Side note: I just realized that this makes it even more baffling that your biology teacher, of all people, made such questionable and misleading statements.