Categorising organisms is a scientific discipline brought to organised form by Carolus Linnaeus (is my nerd-dom obvious?). Is labelling things a biological imperative? Why do we feel compelled to identify things (even people) into neatly labelled boxes?
Sexual identity is one of them. How many of us enjoy a titillating whisper speculating on whether X is gay and does his wife know it? We hear of cases where children kill themselves after being jeered as gay by their schoolmates. Being labelled accurately (or inaccurately) can be hurtful, especially if the label is perceived in a negative manner. The best case scenario is that we no longer attach negative connotations to any labels, but that is pure fantasy.
Nevertheless, is it necessary to let people know whether we are straight/gay/bi/asexual/pansexual?
What business is it of theirs anyway? Unless they are a potential bedmate, I don't see why it would be of relevance to them. Even if you did end up in bed with said person, it is still not of relevance to them.
It may only be relevant if you decide to make a commitment to that person; in which case, I do believe in total disclosure. I see no reason why a gay person (male or female) should not get into a heterosexual marriage, so long as both parties are willing to stick by their vows (To love and cherish faithfully, till death etc etc etc). Sexuality is NOT an excuse to be unfaithful (yeah, Jim McGreevey, this means you); you chose to marry that person, commit to a family life with that person, so you damn well better stick to your end of the agreement, bud.
However, with the famewhore mentality that seems to be pervasive, it appears that people feel a need to let the world know of who/what they are. Look, at the end of it, who cares whom you like to bang (unless you're Roman Polanski)? I agree wholeheartedly with Ms Moreno on her post Gay? Straight? Get Over It. When it comes to treatment of non-heteronormative characters on television/silver screen, I prefer the way the Europeans do it; with a shrug and "let's move on" nod without the drama and obsequiousness that we see in Hollywood.
So unless you're like this guy (see bottom), there is no need to tell all and sundry about whom you'd like to kick the sheets with, yeah?