The fundamental tragedy of 'nice guy syndrome.' I don't want him in my bed, I don't want him in my life, I don't want him angry, so I tell him he's a good guy, hope to never see him again, and he takes his issues to the next unlucky young lady who does the same thing. Maybe I'm wrong, but if someone much larger than me and with lots more testosterone, and sexually frustrated makes it clear he blames his problems on my gender, then I don't feel like I should be expected to point out his personal flaws. He should have friends who can tell him he's doing something wrong.
I'm not sure that would work. Saying the truth about what you think, without any kind of sarcasm or sub-text implying - in a sort of "same level adults" way - would be the best alternative, in my opinion. I understand why you wouldn't want to be the one to say he's wrong, but also, don't say he is right.
I know how much this is a unleveled comparison, so don't take it so seriously:
The same way he tries to apply the "being nice" act, when you "tell him he's a good guy" but really hope to never see him again you are in a sort of "being nice" of your own.
If you give some pity compliment to these kind of guys, they will filter out the pity part and think you actually like them.
drinking the small coffee he bought me = implicit permission to fuck
Hahaha, I would bet 90% of the guys who try to "buy the lady a drink" think they will have implicit permission to something in case she accepts.