It's not natural for you, and that's fine. What may not be natural for you may be natural for others.
I am -- as far as I can tell -- at least a serial mongamist, if not an absolute one (that's difficult to say with any certainty, as I'd have to be in a relationship for 20 years to find out). I genuinely do not have interest in one night stands, I strongly prefer sex after 4-5 months of dating to the "excitement" of the first few times, and become so deeply involved with my girlfriends that I very rarely look at other women.
I am aware that not all guys are like this. I know, for example, that many men are particularly interested in sex with fresh new partners, when someone's body is new and unexplored to them. More broadly, I know that some people like having large groups of friends, rather than small groups of very close ones, as I do. All of that is fine. My point isn't to suggest that everyone has to be like me, just to make sure you realize that not everyone is like you.
I'll make two other arguments here as well. First, it would be a shame if we moved from one absurd absolute (all people should be in monogamous relationships) and swung immediately all the way to the other extreme (monogamy is not natural for anyone and no one should do it). Second, building on that point, given the huge breadth of variety in human sexuality (heterosexuality, bisexuality, homosexuality, transexuality, and a variety of others), it seems very unlikely to me that our romantic temperaments would be absolutely uniform.
I would guess, instead, that some people do best with a huge variety of partners on a rapid basis; others might do best with 1 "main" partner and a secondary and tertiary partner as well; still others might be operate best in a series of short, 2-3 year relationships; lastly, some might truly be monogamists who operate best with a single partner for decades upon decades. Again, given the huge variability in virtually all other aspects of human sexuality, it seems very unlikely that all of us conform to a specific, pre-determined preference for a huge numbers of partners. As far as I can tell, I honestly and genuinely don't fit that description, but I certainly accept that it may describe others, and that's fine.