Yes, this is EXACTLY what Myrtle said 
I’m extremely bothered by that. I already tend to never replay different romances in these games and pick the exact same choices when I replay (I tend to try more things out in visual novels). But both in text based games and in visual novels, I can’t stand when some of the love interests can hook up - between them or with other NPCs. This is literally a deal breaker as far as trying other routes go. I’ll fiercely protect my chosen RO, and totally disregard the others as options.
I guess this part is also true for me:
I understand people who consider the other characters can have their own lives too and not only think about the MC, but eeeh… Even though I understand the opinion, it doesn’t mean I agree.
It would be lovely if authors who write things like that could offer players the choice for these romances to trigger or not.
Still, if I was to go more into depth than being jealous and not wanting to butt in “canon” romances, then I do have more.
Basically, one of my favorite types of romances in visual novels or text based games are romances where that love that was born between the MC and the RO actually SAVED the RO. Be it on an emotional level, or even actually saved his life. I think these are very impactful and touching romances.
But then, if that RO can actually romance someone else, on the sidelines, and be happy too… then what the heck is the point in the MC doing that, and the player going though all the hurdles relating to the romance. It’s extremely discouraging.
It basically downplays everything the MC and the player did.