Actually, from what I can see, there is nothing explicit about a homosexual relationship in Last Christmas. And the video clearly has heterosexual acting couples in it. But since GM wrote it, the gay reference can be inferred. But that can be done for most songs that don't explicitly mention a boy and a girl, and then even some of them.
We'll agree the song sucks, though. But it is at least as much a Christmas song as many great songs associated with this time of year: Let It Snow, Winter Wonderland, Jingle Bells, etc.