I think that, when he said that if he gave you the key, you could come over to his house, that would have been the moment for a puzzled look, and "Why would I want to do that?" Or I suppose a frigid "I'm afraid that won't be possible" could have worked, too. That could come across a bit snarky, though...so maybe just telling him that you weren't really up for chatting would have been better, maybe adding something to the effect of having a terrible headache if you felt inclined.
I guess you would have had to have been there. First, I was really taken aback, like, "Okay, this guy did NOT just say that," and then he said it AGAIN. After that, I still didn't know what to say, so I figured that if I didn't say anything, he'd figure it out and stop.
However, after "three strikes," as it were, he still didn't get it, so it was only then that I told him that I was "giving him a chance to save face." If I'd gone with "Why would I want to do that?"; or "I'm afraid that won't be possible" (although the latter statement was true, because I had an exam that night), then that could have been interpreted as rude or snarky, so I would have ended up in E-Hell over that....if I'd even had the wherewithall to say it. But, it's pretty hard to respond when your jaw is on the floor, KWIM?
Also, if I'd just said "No," then that could have been interpreted as me being snarky as well. If I'd said I didn't want to talk, he probably would have kept pressing me to continue the conversation anyway. As for feigning a headache, I felt perfectly happy and healthy that day, and looked it as well, so I don't think that it would have been plausible.
So, I figured that the best thing to do would be to say nothing at all, but then he didn't pick up on THAT, so I had to finally explain to him why I wasn't responding to his filthy overtures. I mean, okay, maybe I "embarrassed" him a bit, but didn't he think that trying to set up a "booty call" with me, while we were on a public bus, might have been embarrassing for me?
Anyway, after all that, we all had a good laugh about it ("we" meaning me, the bus driver, and the guy who'd been trying to get into my pants), and we (okay, THEY) went back to discussing non-objectionable subject matter. I made it home in one piece, then back to school for my exam later, and I never saw the guy again. Life continued as normal, and I don't think he was scarred for life because I gently let him know that he was being inappropriate.