There's not as much costs involved for a man having sex than a woman, the risks are a lot smaller. Men don't get pregnant, women do. Getting pregnant is a very high cost that men lack, therefore it shouldn't be surprising that men would be more willing to do an activity than women because the cost and risks involved aren't as bad for them.
Not to mention there could be potential evolutionary explanations in the case of natural selection. Natural selection would select males who slept around with lots of women. Men who sleep with lots of women will spread have their genes more frequent in the human gene pool than males who don't sleep around, and therefore you will find that there will be a large share of males (if not the majority) who have a strong urge to sleep around with lots of women for this reason. Whatever combination of genes that causes men to have a strong urge to sleep around with men was naturally selected.
Psychologically speaking I can't say, however all I would say is that as a male it is 100% true to say that to us sex is really just an urge. After I get off I really don't feel any urge having sex anymore, to think about sex anymore, or even think about having any close contact/bond with a woman in general.
Once I get off the state of my mind literally changes and I start thinking about things I normally think about which have nothing to do with sex at all.
Sex is really like an on-off switch to us, once we turn the switch off we literally are done and just don't think about it anymore. This is the reason why I think having a relationship with a woman is a waste of time. The only reason I think it would be worthwhile to be in a relationship with a woman is for sex, nothing else I can think of makes it worthwhile. There's only one girl who I ever wanted a genuine relationship with that was more than just about sex, but she didn't even like me that much and I suspect she doesn't like me, so this option is off the table for me.
I'm not sure with women if it's the same, I'm not a woman so I wouldn't know.
So as for your question about "Why would men sleep around?" Well why wouldn't they sleep around? If your sexuality worked the way I explained, similar to an on-off switch, and sex was merely just an urge, why would you stay with one person if you had the opportunity, knowledge, and power to sleep with multiple people? Some men do stick around with women, and in many cases I suspect it's because those men think that they can't get anything any better, or maybe they think that although it would be better to sleep with multiple women, the transaction costs involved seducing each and every female is tiring and annoying and therefore it is more worthwhile to just stick with one female.
I know females would not like to hear it, but I think the fact is that if men were able to sleep with lots of women, knew they could, and at the same time didn't have to spend a lot of time/resources in order to seduce a woman, then the vast majority of men would sleep around and not be in a relationship with just one woman.