I guess straights since it's harder for gays to find other gay people in their areas so they are more likely to cling onto other gay men in relationships compared to straights who will have it easier to find another partner
I think a lot of it would be 'straight' guys who are bi-curious if anything from what I've seen, but even that's pretty steep. It's all equal chances no mater your sexual preference, as someone has already said "If you're a cheater, you're a cheater".
Gay people are less scared to hide it compared to straight people that most of the time are married, this is why it seems that gay people are the ones who have the tendecy to cheat more.
Judging by the amount of bi guys in relationships with women on the apps I'd actually say on average probably bi guys. Tons of them don't out themselves because it's more convenient for them and they opt for relationships with girls (even if their preference leans more towards guys). What happens is they cheat.