The process of coming out is an inherently political one. It is an act of saying "I am who I am, and I'm not going to let anyone take that away from me." Once you make that leap, it should be no surprise that you'd get involved in countless other ways.
As for why I've always been so political, I don't know, but I doubt my sexuality has all that much to do with it. I've always been this way, at least for as long as I could ever remember. I guess I've always cared about the way things work and making things as inherently fair for everyone as possible. That, or I hate the idea of bowing down to authority figures who think of themselves as authority figures - and not just normal people with good ideas who won an election based on those ideas.
In any event, my point remains: the way gay people vote changes the outcomes of elections, so Presidential candidacy impertinence - saying you're for 'equal rights' but being against equal them - is a vote killer and will only serve to turn more than 10% of the Democratic Party's voting base off. If any one of the Big Three goes out on a limb and truly favors equal rights, they'll very well get the advantage they need in the early primary states, which will give them the bounce they need for the rest of the election. If anyone wants to beat Hill, it may just take the support of the gay population and its preponderance of importance during primary elections.