Describing everyone who isn't straight as "tending against monogamy" is bigotry. I'm aware that there's a 0.1% PP difference or whatever -- still absurd.
This is true in almost all species, as there is an evolutionary benefit for males to mate in excess of one partner, as each subsequent mate increases the chance and amount of offspring. Whereas females have little benefit to mating in excess of 1 partner, as she cannot have more than 1 child in that period.
So if anything I would be sexist against males (of any species)? Or sexist against women? It depends on whether you consider monogamy virtuous or not. Or maybe pointing out any difference, even if morally neutral, between two group of people is enough to be a bigot, in which case I would avoid speaking about protected classes of people, which doesn't sound great.
I don't know what the rules for bigotry are, (but I know it when I see it). But I'd dare say that even pointing out negatives of a protected class can be fine. If we are talking about basketball and I point out that the Japanese team is too short, that's not bigotry. That a comment is tangential instead of central to an argument is probably also relevant.
As for the accuracy of the claim, I won't cite a study, some truths are accessible by simpler, perhaps more social means. If you know gay people, talk to them, ask them directly if they feel comfortable and you care about it. Also take a look into the history of dating apps, is it a coincidence that Grindr was originally gay only? Is there an equivalent Lesbian hookup app?
Also, I'm sorry if you are gay and found this offensive, I just assumed you were straight and were just being offended in the name of someone else. I don't think many gay people would question that men are less monogamous, but I may be wrong..
There isn't an offensive imputation contained in being called non-monogamous anyway, unless you in turn have a prejudice against polyamory etc.