Just happens to be gay gets up my nose because it's one of those phrases like "white, black, yellow or green" that just drips privilege - they are all about the person using the phrase to show how magnanimous & unprejudiced they are, not about, as you say, the lived experience of the gay person in question. For me, "just happens to be ~" is a synecdoche for "they're just like a normal person except they just happen to be ~, and we love them in spite of that. Aren't we awesome for being so forbearing?" Ah, no.