I think it's really common for a man to feel not threatened by his wife sleeping with another woman but threatened by sleeping with another man.
It's common because sex between females is often thought to be "not real sex" because it doesn't involve a penis. On top of that, lots of men get sexual gratification from the knowledge or witnessing of lesbian sex, so if they get something out of the fact their "little lady" is having relations with someone else, it's all good, but if they don't get anything from it, it is an issue. The other common reason men are okay with their women having same sex relations is because they don't feel a woman could replace them as a primary style partner because like, he's a man and shit. So even if the woman does feel strongly for a girlfriend, that girlfriend won't replace or displace him for roles such as co parent or co owner/tenant, because like, women can't fulfill the role that men do.
So as common as this view might be, it is often based on homophobic, sexist and patriarchal beliefs that many of us would not tolerate in a partner.