Only on f95zone! Never change
If it's social, it's natural by definition. What could be more natural than people shifting their desires based on the situation?
Anyways, I guess I'll throw my 2c in the debate. Are we talking monogamy as in 1 partner for life? Or monogamy as in 1 partner at any given space of time? Because if the former, sure, humans are probably much more polyamorous than monogamous. But if the latter, there's no way that I'm buying that the average man (or woman for that matter), ancient past or no, is ok with their current amore dipping into other waters. I'd say multiple simultaneous partners is more often through force or deception.
Arrangements of convenience, politics, or just sexual flings are different; but even then, if there's going to be multiple partners, it's doubtful they want to know anything about each other, much less participate together. All of this is speaking in generalities ofc, obviously there are some unique and interesting ppl out there who don't get jealous as much.
BTW, having said all that, I have zero interest in RL sharing, but I honestly do like virtual sharing in these VNs, especially if it's dominant and there isn't much interaction w/ the other dude (not so bad if he's written as a likeable guy, though). It's like having fun ripping apart a dude in MK while in RL, I'd be absolutely scarred and horrified haha. I'm looking forward to that with Brit. Sharing, I mean... not ripping her apart

Another solid candidate would be Cassie for obvious reasons. The idea of big bro cucking some poor sob that's in love w/ our dear lil sis is pretty hawt