First I need to give a bit of context.
In this manga, the world was attacked by a virus, I forgot the name or it wasn't named. let's call it corona
So basically every people in the world is contaminated by the virus but it has only affect on male who already had sex. All the world's non-virgins transformed into monsters called "Riajuu".
If MC or other virgin guys have sex once they'll transform too.
Near the end of the manga, a new character is introduced: a dude who seems immunise to the virus, he can have sex and not transform.
So they gave him a lot of women to fuck. There is a all chapter where Mitsuko (i think it was her name) has sex with him. But well, you know, he has that ability that every hentai's MC have, he can go like 40 times in a row so she had quite a hard time. And Aina is his "secretary" so she must have already done it with him. Aina and Mitsuko don't seems to like him btw.
Some people may see that as NTR but not me. Anyway he is your typical NTR guy you can see in most eroge.
In the last chapter, MC and main heroine and new characters are in a tropical island with a big hotel. NTR guy and other people come to the island with a helicopter.
NTR guy saw main heroine, he grab her boobs and kiss her with the tongue then they enter the hotel and leave MC and co.
Lastly they said to MC "do what you want we don't care about you" and MC be like "Yes and I'll do this".
THE END
Also, main heroine has been brainwashed so she is like a robot at this point of the story and she doesn't remember about MC. So MC has kinda give up on her and was about to have sex (willingly) with an other woman. Still, it seems MC will try to save main heroine cause he still a better choice than NTR guy imo.
That's basically what's happened.
So now make your own conclusion. NTR or not ?