It really depends on what your criteria is:
1. Is it immoral? I don't think it violates any kind of individual morality for me as long as it is consensual
2. Is it 'good for society'? That's obviously debatable, but
a) I definitely think sex is a powerful act. Sexual desire is a powerful human emotion, like hunger or something. It is what drives us. Toying with it so casually is probably not good for society. Just like toying with hunger can lead to obesity because you're not treating hunger and the human satisfaction from eating with the proper respect.
b) This is compounded with the issues of raising children and families. If you don't plan to raise kids, then this is a non issue. I think we'll probably find out if some of the other ways of raising kids on mass is not too harmful (state raised children, single parent...).
c) Spread the wealth. Monogamy is a kind of socialism in that everyone gets a piece and has a chance at a decent life with a reasonably committed person. This is good for both sexual and emotional pleasure. Remove monogamy, and you're going to have huge swaths of people lacking. In more misogynistic societies, you can have rich men with harems of women, depriving the rest of the men of finding decent mates. Heck, even in the west, the 'hottest' guy seems to make his way through lots of women. Guys tend to be less discriminating that way
Or you can have people lacking the emotional commitment because sex is generally available everywhere.
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I personally think people focus far too much on it being immoral or moral and not enough on its real impact on you as a person and the world around you. Or people sit around saying but this person gets to sleep around, why can't I, or you have women sit around saying men sleep around, why can't I? yes, there are std's and pregnancy things to worry about. But there's also emotions, mental well being, family, society...