For most of people, a game is a game, it's not reality, and the aim of a game like this with a male protagonist is to fuck all the girls you can, without sharing them with any
rival of your character.
I insist on the rival, because that's why most people don't want to share any girl, even if they're not interested by this girl, as you said. Letting a rival getting a girl you can potentially screw is seen as a defeat (and considered as NTR for many people, which in fact depends of the situation, because you can't call that NTR if the dude is just fucking his girlfriend that you're trying to steal from him, it's called voyeurism).
However, some people forget that there're not necessarily rivals in all the games (in TomL, the rival is Vlad, Andrew is a cool guy who's not clearly saying to you that he'll fuck your girls, it's a friend).
A friend in a game like this won't try to "steal" a girl from you, he'll ask permission or help just like Andrew did. So, you have the choice to help him or not.
For Twists of My Life, we can't build a harem, so even if you're interested in all the girls, you have to sacrifice them. So, giving a girl you broke the heart (that's really heartbreaking from Novel when we reject each girl one by one, the faces they did :'() to a good friend, should be great as you can still see more of her and please everyone :angel: But of course, if we had to give any of them to Vlad (the rival), then it's not the same thing.
Anyway, that NTR word is so misunderstood nowadays... no NTR here btw, if we let Andrew with Ann, it's called sharing, maybe with benefits later hehe