How to go about natural interactions
oh just saying i am not opposed to harems or well explained action parts, i am just talking about the forced part, it is just there to say hey i am here so it is this category i.e., like it is a a harem so i am going to introduce multiple characters when the last girl falls for the mc, it's like story is ther for the girl to fall for the mc instead of the girl falling in love during part of the story.
If something feels forced, it probably is. Ask yourself, "would my character really do/say that?" If you want plot to go in a certain direction but your character is resisting it, you either need to change the plot or discover a realistic motivation that would make the character go in that direction.
When people interact in the world, they don't have time to stop and consider their words or actions. They react according to their gut instinct and the patterns of behaviour they've settled on throughout their lives.
Interactions between characters should be multidimensional. In addition to dialogue, don't forget to include facial expressions, body language, actions, and sometimes unsaid thoughts.
Anyways, that was a really vague answer to a rather vague question. If you had a specific example, we could go into greater detail.
Read this link
I like it's explanations on character development.
The reason ecchi harem manga sounds forced is because it is. In High School DXD, why the hell do girls like Issei so much they all wanna do him? He helped a couple, but that's reasons to be friends with him or for them to not think he's a total loser which he is. Girls have a natural resistance to things like harems in the first place, they like the monogamous relationships. They need a reason to join a harem.
1) Real modern harems are usually made by asshole pimps who take orphan or runaway girls, abuse them, drug them, manipulate them, etc into joining their harem.
2) There's the occasional party girl who might get their best friend to give their boyfriend a threesome just for fun.
3) There's also religious reasons—girls who've been raised to believe that's what god wants them to do.
4) Also, a medieval world would make sense where woman are oppressed and believe they don't have the right to argue back to the man.
5) This ones a rarity. An open-minded, free-spirited girl who lets the guy fool around. These relationships usually have boundaries like "just sex, no talking" etc. That wouldn't really be a harem though.
6) A psychological problem with the girl might work. Like she's had a rough life and think's she herself isn't worth anything, then she finds some amazing guy who solves her problems for her.
But a guilt free harem where the girl stays around for awhile isn't very realistic. You would probably have to write half the novel in character development, explaining what miraculous series of events transpired to allow this. A true realistic harem is the type of complicated character development I don't think a single author on this site is capable of right now.
Anime tends to dance around the issues a lot. They make the male protagonist indecisive about choosing a girl. If he did, the other girls would probably back off, respecting the boundaries of the relationship. Or become secret mistresses. The girls one after another go through a miraculous sequence of events that are incredibly improbable. Even if they happened, 90% of the girls would realistically just end up being your friend afterwards.