I think trisexual is just made up. If you're fantasising about animals, you're into bestiality
Wow! I have a fantasist guy that rings every so often who claims he is trisexual, he always asks if he can bring his dog. Is trisexual even a thing?
I used to say I was trisexual - I'd try anything once, twice if I liked it! EDIT: OMG, it's actually a thing. It's actually what I said.
I don't think I've ever had anyone call me and feel compelled to tell me that they are a heterosexual!!
It's an expression. AFAIK it was first used in the TV series Sex and the City, so probably fifteen-ish years ago.