Look, dude, the math is pretty simple. Implying that someone is gay as an insult is inherently homophobic. This is not something confusing or difficult to understand. Whatever you may have felt about Brokeback Mountain (awful movie, incidentally), you’re perpetuating something really ugly. My general rule is if it walks like a bigot and talks like a bigot, it’s probably a bigot. If you don’t want to be taken as a bigot, I would suggest that the best way to avoid it is not to act like one. You don’t know how many people are listening without saying anything. After all, I wouldn’t have said anything until someone else brought it up.
If you don’t care, then say what you want. I can’t stop you from spouting bigotry. I can’t read your mind to decide whether you actually are what you present yourself as. But like I said, using the imputation of homosexuality as an insult necessarily entails that being gay is something shameful. If you don’t mind perpetuating that idea, I can’t change your mind.
I would hope someone who claims not to hate gay people would change their mind on their own, though.
I think the usage in question is one that’s mostly common with teenagers. I’m less than half your age but I didn’t even notice the usage until relatively recently - probably after I graduated high school. But what I’m talking about is the general usage of “gay” to describe things that are bad - it’s not limited to things that are flamboyant or might be stereotypically associated with actual gay people.