For me, it's Sympathy Sex. You know, when you make yourself out to be a real loser and if you find the right girl they'll feel so sad for you and fuck your brains out to cheer you up.
The best sex (well atleast if you are married or in a relationship) is that "welcome home" sex after not seeing the person for atleast a couple of weeks. As soon as you see the person you just want to tear them apart and go crazy! Absence makes the heart grow fonder in more ways then one!