There's much more involved with being straight or gay than who you sleep with. And unless a person is lusting or committing acts which God says not to commit, I can't Biblically find anything that says that God makes it sinful to identify as gay.
Why would it be sinful to call yourself gay just because folks automatically assume you're having sex with the same gender?
There are lots of folks who are attracted to the same sex who identify themselves as gay, but they don't engage in same sex acts.
There are lots of folks who are attracted to the opposite sex who identify themselves as straight, but they don't engage in opposite sex acts.
Now I have looked and looked, but can't find anything in God's Word that comes close to saying that an attraction is wrong. If there is no lust involved, or no prohibited acts taking place, where is the sin?