Because sexuality is so personal, I don't think anyone has a right to say that people should not label themselves as gay or lesbian if they have had opposite-sex relationships in the past. If Cynthia Nixon and Ricky Martin want to identify as gay, let them. Identity is how you perceive yourself. We do not know enough scientifically to say definitively that someone is 100% gay, straight, or even bisexual. But if someone identifies as a certain way, why stop them? Likewise, why get upset if someone eschews labels altogether? Your sexual identity is your sexual identity, and if you do or don't want to be identified as gay, straight, bisexual, or other, then that is your prerogative.
Sexuality is not like race - it is not fixed from the moment we are born. Our race is, certainly. But other things about us can change. Our hair color, even eye color, in some cases, can change, as can our body size, etc. A person may be tall as a child but short as an adult. A person may be fat as a child but thin as an adult. A person may be an outgoing child but a shy adult. Traits sometimes change, and sometimes they just intensify. Sexuality is the same way.
So however someone wants to identify himself, or not identify himself, is fine with me. I certainly don't want someone defining who I am for me. I used to think I was a lesbian, and then I thought I more logically should identify as bisexual, given my behaviors. But now I don't identify as either. I have only loved women and have only been attracted to women, but I have slept with more men than women. More than that, I want to love and be attracted to a man. So don't label me at all.