I do think (well, I may be behind the times by a decade or two) that bi "unicorn" and "versatile" tends to not be about bisexuality itself but a very very VERY narrow target that largely involves M/F couple entitlement to their FMF threesome. Virtually every time I have heard unicorn in particular bandied about, either as a perjorative or otherwise, it's been in the context largely of "how DARE that person CLAIM to be bi and NOT immediately want to eat out my gf/wife as I watch and maybe fuck one or both eventually." Or "OMFG how dare someone who labels themselves as bi not want to form a triad with our already established couple. What do you mean she or he only wants to date one person. What is the point?"
It can be very frustrating. As someone who has dated men, women, couples, ethical slutted, ect--there is a whole fucking hell of a lot of entitlement by people directed towards anyone who claims the bi label but doesn't want to date or screw couples, ect.
That being said. I understand the need for space. I don't think there's really that much of an issue with a MUSH that says up front they will only accept male PCs. Or even more narrowly gay or bi male PCs. It's not like all the players will be men. I do think that keeping women to puppets is a little...well. Kind of offputting, because of my own personal hangups about that. HOWEVER, on a game where there can only be male PCs that is predominantly supposed to be about same-gender relations, but still for some reason allows straight male PCs, I guess it's nice that they can have RP around that if they wish and so I understand why that might be a thing? I probably would not play there because the puppet woman thing is a little much for my sensibilities. Single sphere games are about as narrow in focus when it comes to "we only allow THIS" as I enjoy as a gamer.