Well, I guess it's all very relative. Sure, if it's someone who you got into an open relationship with it might still be a weird request for some people, but yea pretty mild if both are into it. But coming from someone you're in a closed relationship with, that's a different story. Psycho is too strong a word, but it would show that their sexuality has a side I didn't know about and that I certainly wouldn't be comfortable with. If I was in Op's position, I would be very uncomfortable. In fact, like I said I would probably end it real quick if it this sort of thing came from someone who I thought I was in a closed relationship with.
I would be worried that it indicates the person might have some unresolved subconscious sexuality issues and I wouldn't want to be involved any further. It's totally different if said person had been upfront about that sort of stuff and had owned up to it. But it sounds like he's only expressed this desire recently, after a good time together, and is trying to push her in that direction when she clearly doesn't want it. Seems creepy to me. Maybe i'm just not expressing myself well. But yea it can be pretty mild depending on the context.