Yeah, it's optional. The man also made my skin crawl. Sharing/swinging/open relationships/whatever you want to call it is NTR. Again, I'm not against it in general, but it seems off in this game.
You're misunderstanding the concept of NTR;
That involves the guy losing his woman without his consent to another guy (or guys). This is normally a dark fantasy in hentai to humiliate that male character and made to look inferior as his girl falls for another for her own blind pleasure no longer caring about who they were originally with.
A cuckold is one who enjoys watching his girlfriend getting fucked or at least thinking about it if he's not currently present. Rarely involved themselves. Completely allowed and even seen in women (cuckqueans) who enjoy watching their partners getting fucked for a sexual thrill. In the end that partner is still with that person because it was purely meaningless sex.
A swinger/one in a open-relationship is normally one who engages with their partner with another (or more) where they all get off. Sometimes they like to sit back and watch but what makes them different from cuckolds/cuckqueans is that they are physically involved themselves where everyone gets off in a very physical sexual matter.
I think for you, you've blended/blurred the lines of NTR, Cuckold, and open-relationships/swinger culture all together automatically without really investing the time to understand the differences between the terms.
After all, I don't see how you consider, a guy who willingly wants another guy involved so that he can live out a fantasy of double-pentrating his wife or just have another guy watch him pleasure his own wife for the the thrill of being watched.
Now this could evolve over time where the husband may become more of a cuckold and want to merely watch temp fuck his wife but even then the options for him to be sexually involve makes it far from NTR especially since you're not technically stealing the wife against the husband's consent.
From personal experience who has been sexually involved with a swinger-couple, I can vouch that there are more people than you think that relish watching their partners get fucked in front of them out of pure consent or double teaming on their partner together because of what it would do to their partner sexually.
Because unlike the
unrealistic fantasies of hentai, actual typical swinging/open relationships are well developed to a point where in the end it's just meaningless physical sex and that the original couple still love each other as well as before.
Some added perspective for you my friend.