Bisexuality is the attraction to both men and women. However the problem is that people have this idea that if you're dating men or transmen if you're a woman or dating women or transwomen if you're a man that means that you are straight. The kicker is that attraction doesn't just shut off when you enter a relationship. A woman who is dating a man or a transman can very will still be attracted to a woman or a transwoman. The same can be said for a man that's dating woman or transwoman who is still attracted to men or transmen.
And that's what it boils it down to: attraction. The reason why a bisexual can't have both a girlfriend and a boyfriend or a husband and a wife are two reasons: A) People see bisexuals as being "greedy" as if we're hoarding away people like they're treasures. and B) Monogamy is enforced majority of the US. Majority of society looks down upon polyamory. Or C) the real issue is that you're just biphobic.
Bisexuals will forever be apart of the LGBTQA community. There's no point in people who are lesbians or gay trying to say that bisexuals who are in a relationship with the different sex that they're straight. Those attractions to the same sex don't go away and aren't going to be away. What needs to be done is for people to stop trying to alienate bisexuals from the community.
Bisexuals are also hated by the moral majority. Bisexuals deal with getting kicked out of their homes or being raped if not at a slightly higher rate, under trans people. They're looked as sinners and are treated the same as the rest of the LGBTQA community, so if they're straight wouldn't they be accepted by the moral majority?
And that is where your biphobia becomes an issue. Bisexuals aren't straight and were never declared as straight. They are just as oppressed as lesbians and gay and are out here trying to live their lives in peace like everyone else. The best you can do is to support them, not ostracize them.