I have a few theories on this, drawn from a mixture of personal experience and observation. I'm a straight male, by the way.
Straight men generally see women as potential partners, and so they imagine whatever is said, is said by their partner. It might be perfectly appropriate to say to someone that they contribute to rape culture and that their previous encounters may have been rape, if you're speaking from a point of concern for society. However, in the context of a relationship it gets weirder. Someone you're with telling you your past lovers didn't actually want you is doing it out of jealousy. Someone you're with telling you that you contribute to rape culture either needs to get out or is trying to gain power over you. So if I have the minute possibility that this woman might like me in my head, it's easy to consider her saying this if we were in love and see it as an attempt to bring me down to submission.
The second possibility is this: as socially savvy people most of these women who are talking on the internet about consent and rape probably would change their tune if they wanted to sleep with the men they're talking to. No one would dare telling a stranger they want to sleep with that they should ask for permission. So what for most is just a suppression ("I want you to sleep with me, and there's a chance you won't want to if I kill the mood talking about rape") is seen as discrimination ("I find you repulsive, so I'm going to tell you that you're part of rape culture").
Third, and I imagine this is the most common, most are aware that most are fine with reading social cues and bringing it up verbally if there seems to be any misunderstanding. To an average person who just doesn't care about this issue and has never really thought about it, it just seems weird that someone's coming in trying to dissect their sex life. Someone who has never been on Reddit before would likely feel weird about how obsessed Reddit is with sex. They'd be more weirded out by how obsessed Reddit is with rape. "What the hell? Why is everyone trying to make all these rules? I just came here for a burrito coupon. I am never coming back here, this looks like a horrible place".
Since #3 quickly leaves Reddit, I imagine the most common are #1 and #2.