A few weeks ago, I had an exchange with someone that left me befuddled.

During a conversation, I mentioned that a girl in one of my classes during college tried to influence the TA (who did the grading) by hitting on him, to the point of offering to sleep with him (though he refused).

At which point someone who was part of the conversation bemoaned the "sexist system" that allowed things like that to happen. It was a lighthearted conversation that had nothing to do with the politics of gender, so I was a bit surprised when she said that.

I asked how it was sexist. She said that if there wasn't a "sexist system" in the academic world, the female student's tactic would not have worked. I asked her to clarify if she was saying that it was sexist against the student, and she said yes. I asked how sexism was to blame for the female student trying to use sex to improve her grade, and why the student alone wasn't responsible for what she tried. She looked at me like I was an idiot and said "do you seriously not see how it is sexist that a man will improve a woman's grade if she has sex with him?"

I was too shocked by the sheer lunacy of the claim to respond. What would you have said in my place?

EDIT: To clarify, I'm asking how you would argue against the claim because there was someone else present and I didn't want to cede the argument. If it was one-on-one, I would certainly have given it up as a lost cause. If you're trying to convince third parties, I think it's important not to let a wrong argument win.