I would prefer to play games that are GOOD, even if the MC is gender-locked or sexual orientation-locked to vectors different than mine, rather than playing average games that either allow me to pick my gender or present things from the perspective of the proverbial "Ageless, Faceless, Gender-Neutral, Culturally Ambiguous Adventure Person".
Three really fantastic ChoiceScript games (A Study in Steampunk, Sabres of Infinity, Guns of Infinity) are gender-locked (although not orientation locked.)
I note that no one seems to have published a CS game that is gender-locked to a female protagonist. No one has published a CS game that is gender AND orientation-locked (like, for instance, Nicky Case's non-CS Coming Out Simulator.) I wonder why that's the case? It certainly doesn't seem like there's a dearth of talent or people who either have direct experience in such things, or at least the ability to be very empathetic in such things.
If one of the side objectives in writing a CS game is to get people to open their minds about different people, lifestyles, genders, choices, getting them to put themselves into shoes of someone quite different from them is probably the best way to do it. I'll be honest, every time I play an RPG of some sort and am offered a choice, I will naturally pick "male" and "heterosexual" the first time I play it. It's easier. Games are for leisure and I don't want to have to think too hard about things. (If I liked the game, I will probably go back and try other paths to see what's different.)
I've played some darned good CS games that tell a story from the perspective of MCs that allow gender choice, too (Slammed, Choice of Robots, Choice of Alexandria leap to mind; there are others.) But you know what? None of them moved this cynical old Gen-X'er on issues related to sexuality and gender the way Nicky Case's gender-locked, orientation-locked game above did.
So the answer the original poster's question: decide what you want to accomplish and go forward from there. There's nothing wrong with letting the player choose their gender etc. ala an action adventure like Mass Effect just to get them immersed into the real action (which really isn't about gender.) But at the end of the day, it's your art, and if you actually want the piece to tackle social issues related to sexuality, limiting player choice in this instance may actually make it better.