What Arica said, and also, you don’t know her wife. She may be bisexual. And while I have no way to support this, I personally believe that most people could be attracted to at least a small number of people of the opposite sex they are usually attracted to. If you’ve ever looked at TV Tropes, they call this “If It’s You, It’s Okay”—an otherwise straight person meets one specific person of the same gender as them (or gay/opposite) and is attracted to them simply because they are a perfect match for them in terms of personality. Some people would be uncomfortable with this attraction, but others would accept it and be willing to be in a relationship with the person. It could be either of these things (she’s generally attracted to people of both genders, or her wife is an exception to the rule).