Well, there has been scientists that hypothesize that the reason that males are more willing for any form of sex is related to evolution. Since men just do the deed and walk, feeling relieved, women get pregnant and carry your child for 9 months(in nature). So, over time, it's kind of engraved in the mind of most women to pick more carefully for a mate, since it's a long process afterwards.
Basically, men don't get pregnant, just sex then walk freely... Although condoms and birth-control change that, it's still kind of subconsciously present since it has just been recent years we've had condoms. Not to mention there's still that chance that they can get pregnant, it's a bigger choice.
Sex is associated with pregnancy, pregnancy is associated with 9 hard months of carriage and 9 hard months of carriage are associated with "I better make it a good'un!"
Men were always free to be whores, whilst women have had to make it a big decision.
As for casual sex, I think we'd all like to know... Today you need to be in a relationship to have sex... Which is a much greater thing, however, it'd be nice to be able to get casual sex every once in a while.
You could always find an easy lady, but usually people don't want to touch them