My guess is that one of his "girlfriends" exclusively wants to be tribbed, and the other one exclusively wants the duck, so Chris is going to be the "bisexual" who can bang women in either the male or female role.
And because he is dumb, he won't think to ask what these "girlfriends" did with each other before he came along.
The issue with that theory is that was already how he thought himself once he came out as transgendered. He made all kinds of comments along those lines. About how he was a great partner for a woman, since he was a woman but still had the duck. So if it was really about having sex with two different women in two different ways I don't think it merits a big pronouncement.
He has also made various comments over the years indicating that he understood what the word "bisexual" means. For him to misuse as badly as that in a public pronouncement without a rambling explanation about how this doesn't mean he is attracted to men would be unusual.
Here are my completely speculative theories:
1) He was blackmailed/tricked/trolled/bribed into making the announcement, and it means virtually nothing.
2) His new "sweethearts" include a male and a female. They have persuaded him, most likely because they are trolling him, that he needs to be with the dude also if he wants the China. Maybe not actual one on one sex with the guy, but in a threesome or something like that. He has to profess his bisexuality to get the china, although I doubt the china is forthcoming.
3) This is pretty close to your theory, but I think a little more likely. His new "sweethearts" include one or more women who is biologically female but presents as a man, at least occasionally. (Again, I doubt this would be real. More likely trolls pushing Chris' boundaries.) Chris has been persuaded to acknowledge her as a man in some of their interactions, making him bisexual. So it's not that he is taking both a male and female role, it's that his partners are in some way. Which is a key part of bisexuality that I don't think even Chris could fuck up.
4) He has been persuaded in some way that true and honest open-minded members of the LGBTQ community should be open to relationships with all types of people. He has some twisted explanation in his head about how he is now a bisexual who is only interested in women.
5) He has actually discovered an attraction to men and has decided to declare it. He plans on pursuing relationships with men as much as he has done with women. I think this is the least likely explanation, but I suppose it has non-zero probability.