Women are most attracted to men who display an attitude of “If you won't sleep with me, that's fine, I was getting tired out from all the other girls anyway”.” PapaBear3625
You hit that nail on the head. Women are notorious “mate poachers” - they want a man who already has a woman. He is vetted, domesticated, and desired.
They did a study where women answered questions and then were told that based upon their answers they were emotionally compatible with THIS guy (the same picture of a male model). Half the women were told that he was single, and would they like him to contact them? The other half were told he was in a relationship, but would they like him to contact them?
Of the women told he was single - only 60% wanted him to contact them. Male model, told that he was emotionally compatible - and 40% had no interest in him contacting them.
Of the women told he was in a relationship - 90% wanted him to contact them.
That is why men's sex lives seems to be either feast or famine. When you are hungry - nobody wants to feed you. When you are satiated - women all seem to want to whip something up in the kitchen to tantalize your taste-buds.
I think a large part of it is the assumed pre-vetting. The guy in a relationship is assumed to have good qualities, otherwise his girlfriend would have dumped him (and this is judged relative to the hotness of his current girlfriend). A guy without a girlfriend is judged to be a loser, by the same criteria.
One possible way out of this for single guys, is to be seen hanging out with a cute woman.