It might depend on where you look. I have a friend who is large but found a ton of dating options on the dating websites she chose to use. She even married the one of her choice - and she had many choices.
On the Biggest Loser, I think it's not how you look per se, but how you feel about yourself. Poor Koli (or is it Sam?) was csaying he's dated before but pushed people away, b/c in his words, "no one would want to marry a 400 pound man." He's a nice looking guy as is, but has zero self esteem.
Michael, starting at 526 pounds, is also a nice looking man, and seems very warm and fun to be with. There is no reason he can't have a loving girlfriend or wife. If look at a lot of these people, they are pleasant looking at least for most of them, and even pretty right now (like Michelle/red, Sunshine/yellow and Stephanie/purple).
A lot of these single people admit to having been heavy most or all of their lives. My guess is their self esteem is in the toilet.
As Lisa pointed out in another thread, the contestants seem to be glowing by the time they reach the 'makeover' episodes, because they feel happy and energetic. They feel very proud of what they have done. Even the ones who are not 'skinny' look thrilled and pleased with how they look. So my sense is that it's not that there is extra weight kicking around; it's how they project their self image.
Does that sound about right?