Well Mia's character has changed a lot since the early rpgm versions with the doggy in (for those who did not play that game then, it was way more believable that Mia after some cooldown after that rape scene, was quite happy to have sex with her brother just cause he was a good lay).
She changed even more in game towards the end and this is a new game that is intended to be more romantic as swm1, but in the later versions she had no scenes in the main canon game where she was fucked by two males at same time. So even though she was quite wild she did show a change in her stance to sleeping around when fell in love with MC and second she did not have a fetish shown in game that is really a problem for me romance wise. Now I also would prefer not to see scenes with Grace in game younger or older before she was with MC, but if it had been a one on one scene where she fucked because she had an itching pussy I would not have had any problem with it at all beyond I rather would not have seen it, but no effect on her being romanceable. Others would though, it is just the way it is with peoples likes and dislikes.
Now I do not know your fetishes and kinks, but just assume in keeping with original swm1 you are not into beasties and in next two coming scenes she blows off and fucks a dog first and the one after she does same with a horse? Would you still want to romance her after if knew that even if 20 years ago you knew for sure she did that last and no idea if she is still into that (or at least fantasizes about it as something sexy and hot now or else she would have chosen another fantasy to tell you), but why would she not be since those kind of tastes do not tend to change? For me I might see MC fuck Grace once or twice, but a romantic relation no. Well I am there already after the double MMF scenes.
Players who tend to immerse and put themselves in a game, tend to not like to be confronted with fetishes in their main ladies that they dislike in reality as well. While if you do not immerse or like the fetish it is way less of a problem
Now in this game Mia has a boyfriend so yeah it is likely they had sex and no problem with that as long as not shown in game. I will even survive actually if shown in game and she will still be romanceable for me with that, but I know that for quite a few others that would not be the case. Now dev should do what he wants, but also be aware that it will have that effect on them.
Hope that answers your question and will try to stay away from this discussion further since I made the point I wanted to make already a few times too many by now in discussions with players.