I wouldn't say I always enjoy more than one sexual partner at a time. I more so, just enjoy sex lol. Based on my own experiences, every time I have been in a casual relationship, we weren't always having steady sex. Some weeks we would have sex 6 times in that week, or some weeks we'd only have sex once or twice in that week, or maybe we would even go 2 weeks without, but still be hanging out as often as possible. If I'm going out with friends on one of those weeks where I haven't had sex in 2 weeks, then I may find it intriguing to take someone home with me.
It may also be as simple as I just didn't care enough about those people to stay monogamous! I am in a committed relationship now, where I would never cheat, and we've been together almost 2 years. I have also been in a 3 year relationship, where I never cheated. In the first relationship, I never had a desire to cheat, but I did go a bit boy crazy when we broke up. With my current boyfriend, I did come close to cheating (blue eyes, if you recall), however I DO love my bf enough to have come to the realization, that I do not want anyone else, so I broke off that "friendship." It helps that we have sex on the regular, but even during times where we've gone days sometimes a couple weeks without sex (not very often) I never wanted it from anyone else. Like I said, it could just be as simple as me not caring enough for those past individuals, to stay monogamous, but who knows.
If it ever reaches the point in my current relationship that I no longer want to stay monogamous with him, then I would break up with him, because I know that is all he wants. If we ever got to a point where we stopped having sex "just because" then I would also probably leave, because clearly there is something wrong in the relationship, if we cannot be intimate with eachother anymore.
Again, I'm not saying I would never TRY a polyamory relationship, but I don't think it would last simply because I AM selfish, and I DO like attention, and I don't know that I would like my SO giving his attention else where. But at the same time, I would have attention coming from multiple partners as well, so who knows lol.