Because his "girlfriend" is a guy named Bruce.
If you are a girl or a guy and there is a guy in your bed and he inserted something of his inside of any of your body's openings during the night, and has been at your place for more than an hour or a day, then he could be your significant other, otherwise, he just may be a male prostitute, or the one you met at the bar last night
Joe: "Hey guys, Dan has a girl staying at his place. I have been to his apartment twice this week and she was there both times. And both times she came out of Dan's room with only a towel wrapped around her and she is a blonde. I think Dan has a significant other now"
Everybody: "Yay!!! Way to go Dan ... Score!!!"
Dan thinking to himself: "I wish! She is just a really cheap hooker"
Gina: "Hey Jane, I saw you with a guy last night. Was he a male prostitute?"
Jane: "No, he is my significant other"
Gina: "Really? because I may know something about him that you don't seem to know. His name is Ken and he is a male prostitute. I have hired him to get one of my girl friends laid one time not too long ago"
Jane: "Okay fine. He is not my significant other. I hooked up with him at a bar last night and we just had a one night thing"
Gina: "Did you guys use protection? I mean with him being a male prostitute and all, it is a pretty scary thing to hook up with someone like him at bar and bring him home"
Jane: "Okay fine. He is not my significant other, and I did not hook up with him at a bar either. He is a male prostitute and I hired him because I haven't had anything inside of me in such a long time. There you have it. Happy?"
me: no. he's my significant other.
karen: what's that?
me: we only see each other but we don't have the official title. i.e. he's my significant other.