Oh, have you once entertained Bisexual Thoughts? Yeah, sorry, no matter how many seconds or years you've been a lesbian, you're still not a gay man. Which is the specific thing you're appropriating.
Sorry to rain on your parade -- through my rainy-day lack of civil rights, and your fun experiment in being oppressed -- but that's how privilege works. And yes, screaming about that just makes you look like more of an asshole, which is also how privilege works.
Update 1: The thing is, I get that a woman's sexuality -- a person's sexuality -- is a moving target. There is not a person I love that hasn't fought this fight, and I mean that literally. I don't have a family member or a friend that hasn't come into this conversation. What that means to me is, You fight your fights and don't fight your not-fights.
If there was a reason for me to claim bisexuality, I too could write down the dates and times I wasn't exactly gay. There's not, because bisexuality, especially among women, especially provisionally, claims privilege.