I wouldn't be surprised if it's more common than you expect, which is basically none. I sure the women talk about stuff they don't want their husband to know, and they could be pretty close. Throw in some stress and people start fucking. It'd stay a secret, so I don't know that we'd know how common it is, but (as a straight man) if I was in a polygamous marriage with one woman but a bunch of men, is seek companionship with the other men in some form at least.