I won't defend all the Harem Builders, but it isn't ALWAYS like this......
2. I assume you want the option of romantically pursuing any female on screen. IE. Most players don't like it, when a female character is NOT dateable. They HATE it when some other male character is or can date a non available female character.
..
Consider a situation where your favorite character isn't a LI.
You might prefer/identify with character over all the other ones, and you are hopeful that the dev just hasn't gotten around to adding that particular character to the "stable" yet.
In that case, you might not want to see that character fucking someone else.
Not just because of the sex, but because it likely means the dev has now "confirmed" that they are not a LI.
Now expand that to an audience where EVERY character is someone's favorite...
It is unavoidable that some people will be disappointed.
The only real way a dev can avoid it, is to make every single character a possible LI, which railroads the story into a full harem.
The story part is why many devs DON'T do it.
But it doesn't mean that everyone's arguments about a specific character have to come from a "I want them all for me and my rivals must all be celibate" argument