Infidelity is a very rare theme in gay erotica. The majority of fiction out there are serendipitous encounters between single men or just the initiation of a new relationship rather than a continuation or, and in the case of NTR, a violation.

For something to be NTR, there has to be a notion of "us" or "them" depending on story's point of view. For example, if the p.o.v. character watches his crush kiss someone else, there was never any "us". You can't lose something you never had. However the p.o.v. character could then steal their crush away from that someone else, then it's NTR. It violates "them", see?

I'd say the weakest form of NTR in gay erotica is hypnosis or the molestation by some tentacle thing. It's weak because there's not a strong sense of "us" between the victim and their significant other. It's impersonal and their relationship isn't changing.
There's a degree of mindbreak, where the victim accepts their situation and become a willing participant, but that too is an illusion of control. Unless the significant other is chained down or left in the dark, their's no emotional punch to empathize or restriction keeping them from getting in on the action and maintaining "us".