I just googled netorare and it seems like the defining features are (a) the cheater is in a loving relationship with party a, (b) the cheater initially dislikes party b, and (c) the cheater is forced / coerced or otherwise not initially into sex with party b but winds up liking and continuing it, often without party a finding out. 

So it's like a cheating fantasy, whose readers are either playing into their own insecurity about their partners or getting their rocks off about "turning" someone else's who doesn't like or want to fuck them.

Accidentally voyeuring your crush having bad group sex is some weird relative or offshoot of cuckoldry but I think it must be so unlikely and specific as not to have a name.