Fair what does that even mean? life was never fair and probably wont ever be fair.
2/3. MC should be loyal to his girls, but men show loyalty different than women, they pour their resources and time into their women and also protect them every way necessary.
Ideally MC gets the green light from his women or he makes it clear from the get go that there are different women (lust drug, he needs the outlet) so that the LIs can make an informed choice <- source for tension and story progress for the writer.
If he's simply cheating on all of them he's just a scumbag.
Edit: Dev if you're bored or have 30 minutes of free time take a look at the first 5 pages of this threat.
https://f95zone.to/threads/love-and-temptation-ep-2-s2-gruboop.147940/
There the dev just mentioned the possibility of a poll to determine if a LI would sleep with someone else, now imagine what would happen if such a scene gets sprung on the players later down the line when they are invested in your game.