I think because those single guys don't know or realize that the rewards that the Lord will give us will be so much better than sex.
It's like when we were kids and found the opposite sex "icky". "Eww! I'd never want to kiss <insert name of person of opposite sex>. I don't want their cooties!"
Then we all get to an age where all of a sudden, the opposite sex wasn't so "icky" and that union with them (whether the approved kind by God through marriage or the unapproved kind otherwise) was discovered to be fantastic. We think, "Ahh, nothing can possibly be better than this!"
But when all of this age is said and done, God WILL reward us with better things, so much so that we'd think all those rewards will be better than sex.
....And no, I'm not talking about the pudding dessert!
❧