You are talking about two people that you are having relations with, who have a prior relationship, sharing a kiss. That is all that happened between them sexually from the time you met, one kiss. I am sorry that triggers you, but there is no reality in which that is NTR against you, and even if it were, in much of the world, a kiss is little more than a greeting, like a hand shake, it's not a big deal. Any sex they had was before they met you, so it is not NTR, except maybe against Tsug. People you are with, having relationships before they meet you, is not NTR. People you are in a relationship with also caring about other people is not NTR, it's just being human.
Yes, if you are fucking both of them (or something similar), that makes it okay for them to kiss, and hug, and maybe even more, because we are all part of the same group anyway. From a sexual history PoV, if you are fucking them both, they ARE fucking each other anyway, just without the fun, with you as the middleman.
This is YOUR example of the husband, and in that example, you said the husband kissed the wife, who you were playing with, in front of you. Megan was in a relationship with Taina before you, so yes, she is the 'husband' in this example. You are the 'other man' the wife is cheating with ('husband' too). We might not have known that, but that is the fact of the matter. Yes, her backstabbing hurt, but NTR, it was not. Taina and Megan were cheating on each other with you, not the other way around.
This is even less of an issue when you consider it was a marriage of convenience. Even in the current era, most political marriages have concubines for both parties. It's pretty much expected.