Homosexuality vs Transexuality?

Well, as far as I understand, homosexuality is the preference for sex with the same gender, and transexuality is the sexual identity with the opposite gender. I read (though the conclussions were not definitive) that homosexuality happened while on the womb (at less for guys, some issue about hormones, making a boy effeminate). I understand that could work for transexuality, but no for homosexuality, but I met lot of homosexual gays that looked manlier than lot of straight guys. So what would you think? both homosexuality and transexuality share the same origin? and if that is defined at the womb, it does happen the same way?