It's a paraphilia: a weird, out-of-nowhere kink that doesn't necessarily define the person's broader sexuality. Same thing as if you have, for example, a guy who's normally straight and attracted to women in general, but who's
also sexually attracted to chairs. He's not suddenly chairsexual or bisexual for it. He's a straight guy with a paraphilia.
IMO, chasers fall into two categories:
- Heavily closeted gay guys. Those are the ones who chase shemales to suck and get fucked by them. They're attracted to men but can't/won't accept it so they go for the closest thing to a man that they deluded themselves into thinking is not a man. Those often don't really care about appearances and so end up as bottom feeders picking up those who gave up trying to get a woman to fuck them. Those guys are gay and will always be gay.
- Otherwise straight guys with a paraphilia towards dicks. Not men, just the dicks. Otherwise still fans of round asses, tits, delicate and feminine features, all that. Those are usually the ones with the dominating streak who chase after the really girly-looking troons out there. Once the troon in question gets the snip, though? These guys lose interest and go find another girlyboy, or go back to women.
You could absolutely call the second group bisexuals, but the range of attraction on the male side is so limited that it feels weird to me to use such a broad label on them. It's like calling bisexual a guy who likes most types of women all the time but also sometimes feels a mild attraction to... let's say... the guy who played lead on The Witcher. No other man, just that specific guy.