what's the point of the subway scene

I think the scene might want you to judge their personality especially Alex. Whether she should be raped (Sounds crazy, but if a girl is *beep* up like Amy in Gone Girl, why shouldn't they get raped), or whether you should feel compassion about her. The subway scene potrayed deeply when they're together and this will reflect the scene you just watched minutes ago. I know they're stupid to discuss the topic out loud publicly, but that's what the director wanted it to be. To depict your judgement of what you just witnessed the aftermath was tolerable with how their life were.

The other thing is because if you listened to their script meticulously, they said things like the need to focus during sex, the beauty of orgasms, the best man wins etc.

At one point Alex said, 'a man needs to focus on himself, not on pleasuring the partner. If a man needs to pleasure his partner, what he needs is to focus on his body motion to get his partner to feel what he's feeling.'

I knew when the dialogue came up, the first thing that bubbled in my brain was the retape of the rape scene. The man really pleasured himself until he had a good orgasm, so the subway scene also potrayed that she should get the pleasure even though she was raped intensely in a red light tunnel.

So the subway scene had two reasons.