If at least one of them is fixed, at worst it's just a show of dominance. The bigger concern is for the dog humping. Those shows of dominance can lead to aggressive reactions from some dogs. If your dog isn't the type to react aggressively, it's not a big problem outside of any social awkwardness. The problem would be if that dog humped another that really didn't want it. Then there could be a fight. So it's in the owners best interest to discourage that behavior lest they hump the wrong dog.

All that said, even if your dog isn't the type to react and you just don't want another dog humping your dog, you should be fine taking some action. Either avoid the other dog or if that's not possible keep between your dog and the humper. Even physically holding the other dog away from yours should be fine. If the other owner takes offense to this, you can explain that you don't want their dog humping yours. At this point, they'll either concede and try to do better, remove their dog from the situation, or devolve into full Karen. Up to you how to handle that.