Advocates for homosexual rights argue that homosexual tendencies are something that people can't help; they're born that way, and therefore we have no right to prevent them from living the way they are.
However, many of the same people will argue that people can choose their own gender.
On one hand, homosexuals do not differ from heterosexuals in any observable biological or mental aspect. On the other hand, men and women differ in numerous ways on biological, mental, and emotional levels. How then can one argue that gender is open to choice while homosexuality is an unchangeable characteristic of a person?
"Woe to those who call evil good and good evil, who put darkness for light and light for darkness, who put bitter for sweet and sweet for bitter!" ~Isaiah 5:20