The only reason you think it's dipshit behavior is because you are focused on the loser end of it. I've played "friends" that I have lost 6 sets in a row to because I knew I had the best of it but was just playing bad. Then, the next time we played, I win the first set and he quits. Next time same thing. 3 more times same thing. He wouldn't play me anymore after that. I was stuck on set over a period of time.
Did I like it? NO. But, I don't hold anything against him for it. It's my fault for losing 6 sets in the first place! Now, granted, in this case the loser quit. But, did I ever get a fair chance to win my money back in your eyes? I'm sure you would call him a nit. I don't.
What you and others fail to understand is that the OP's opponent stood by the bet. The bet was for ONE SET. The opponent won, and he DID play another set. Which was a totally separate bet, which he also won. You guys make it sound like the bet was not for a set, but for as many sets as you want to play. That's what is ridiculous.
If it takes you a while to warmup and get your head in a match, then don't play short sets! Make the set to 20 or 30 games! And, quit crying when you lose!