Choice vs. Nature (and something about homo-dolphins)

I’ve had this post half-start/half-finished for some time but a recent post over on, FriendlyChristian.com, prompted me to finish this thought all the way through. My problem lies specifically in an argument used, implicitly or explicitly, by many pro-gay activists and bloggers. That argument is the following:
Homosexuality should be accepted because no one chooses to be [...]