I think there is a difference between teasing and being serious, so your example don't really work here. Whatever Usagi says was put in her mouth by scenario writers, and when they obliged her to say to Fiore that Mamoru is her boyfriend, for sure they knew why they added that scene in the series. You can't analyze actions of an anime character as if they are a real life person who has a will of their own.
Okay, I really don't know what those examples have to do with the rest of this, so I'll just go back to the core issue here.
When it comes to a work of fiction, you have to separate information-as-revealed-by-character from information-as-it-really-exists. Looking at the world through Usagi's eyes will give you a world that is very
different from a world seen from my eyes and one seen from, say, Setsuna's eyes, and all or none of those could be the world as it was meant by the screenwriters. The screenwriters, yes, wrote a line that means that Usagi thinks Fiore is gay. This means that Usagi thinks that Fiore is gay
and absolutely nothing more than that.
In order to determine what the screenwriters actually meant about the character, you have to look at EVERYTHING about him, not just one character's opinion. Is there evidence that he is? Yes. Is there evidence that he isn't? Sure. Is there anything actually definitive
, like, say, Fiore actually outright saying, "MAMORU I AM IN LOVE WITH YOU BECAUSE I AM ATTRACTED TO MEN"? No. There is not. And that's what we're left with here, fan speculation.