Hmm, I wonder how you'd feel about this situation, OP.
Scenario: John and Mike have been best friend for years. They frequently talk about their lives and are always honest with each other. One day, they are having a man-to-man conversation about John's girlfriend, Susan. Here's an excerpt...
John: Fuck, Susan won't even let me do anal! >:(
Mike: Dude, if I were your girlfriend, I'd totally let you put it in my ass. No homo.
John: Thanks, John, you're a real pal. :)
I think the usage of the term in this context can possibly been considered appropriate, as Mike may just be saying how loyal he'd be to John as a girlfriend, which parallels his loyalty to John as a friend. Then again, this may actually be Mike's way of saying that he wants to be John's "girlfriend" or simply wants John to fuck him in the ass. It's open to debate.