Glad you're here.
I basically agree with the others that it's not Guru's place to tell you what your sexual orientation is (nor what it "will be"). You shouldn't be pushed into any situation you're not comfortable with.
I would tell Guru that sure, it's possible that any one person may eventually discover a bisexual side within themselves, but that doesn't mean you're one of those people, and he shouldn't assume you are. Tell him his expectation of you is making you feel uncomfortable, and that you need to know whether he can handle a permanent V situation.
Some of this may take awhile to play out, and you may have to see what you can live with, what he can live with, and of course, what Farrah can live with. Does Farrah know about this bisexuality expectation? How does she feel about it?
I hope things smooth out a little more.
Love means never having to say, "Put down that meat cleaver!"