Ich bin nicht schuld! 's ist Gottes Plan!
Picky man. 
When he sees Carol have sex on the first date with Mark, he doesn't want to see it that way anymore. If he applies the same logic to their situation as he does to his own, he's forced to conclude that Mark is a much better guy than he is; after all, he got Carol to see what an amazing find he was after only one night. So he takes a shortcut to feeling better - he decides that Carol gives it up to easily, doesn't look for a guy who is worth having sex with, and is really just a slut. That way, Bob can still feel good about his "accomplishment" without feeling inferior to Mark.
Yes, that's exactly it. Actually, this fits nicely into my metaphor from post #2 about 'playing chess with pieces that move around of their own accord': Bob loses to Mark not because Mark is a better chess player, but because the pieces want Mark to win. Of course, this feels unfair and frustrating.
Mache dich, mein Herze, rein...
@Lawyerdude - ha ha ha.
@jonnyfog - I'm being unfair to ugly dudes, is that it?
@Iaculus - I never said it was loathsome, but yeah, I look down on people who use sex as a means to an end rather than an end in itself. If I ever figured out that a girl was having sex with me just to ride in my sweet car or eat dinner at nice restaurants I like to go to, but didn't actually enjoy the sex itself, I'd drop her in a second. I wouldn't hate her or anything, but we'd have a conversation along the lines of "Well, it's been fun, but I'd rather sleep with a girl who actually likes sleeping with me, thanks. See ya."
And as to long-term vs. short-term - I don't think that sleeping with someone just for the financial security is a very good long-term solution at all. Committed relationships should be about love, mutual enjoyment of each others' company, and yes, great fucking sex!
edited 10th Apr '12 2:37:27 PM by Martello
"Did anybody invent this stuff on purpose?" - Phillip Marlowe on tequila, Finger Man by Raymond Chandler.