Do you believe that non-marital, non-love-based sex is immoral in itself?
Sexual intercourse is a biological process. Other than asexual reproduction by some simpler animals like bacteria it is pretty much a fact that every other living thing on earth wants sex, seeks sex, and tries to get a sexual release. Many species establish expectations and practices around it to try to make reproduction orderly. The ability to get a release without making an offspring (whether that's masturbation, or in the case of humans contraception) is just one more part of trying to have some order around what is otherwise a messy sloppy practice by a species. So, neither wanting sex, nor having sex, nor getting your release, are in and of themselves immoral.
For me if neither party/parties are cheating it's totally cool.
If they are cheating... Fuck them :( SEE WHAT I DID THERE THOUGH...
Sorry but sluts are going to be sluts whether you preach abstinence or give them contraception. Better societies have pro-choice and contraception to maintain good, happy population rather than repressing what's natural.
Some people are very horny, some not at all. That's the way of diversity.