As far as I know, it really isn't. There are a few things listed (or suggested) as sins in the Bible which
involve homosexuality, but only to the extent which the things listed as heterosexual sins involve heterosexuality.
Lust is, supposedly, sinful regardless of whether or not the object of desire is the same gender as you or not. It's the
lust, not what it is directed towards, which makes it sin. Every example I've ever seen given by people claiming that homosexuality is intrinsically sinful is
actually talking about something else.
The distinction is subtle, particularly since there has been a long-standing cultural taboo against homosexuality, but to the best of my knowledge it's never homosexuality
per se which the Bible is talking about: it's invariably some form of lust.