Within the context of Christianity, most conservative interpretations say that homosexual activity is indeed sinful. This is due to multiple passages in scripture, both OT and NT, describing it as such.
Note that this is an interpretation that is internal to the Christian religion, nor are all those who profess Christianity in agreement with it. It is, to my knowelge, the majority opinion within Christiandom however.
Regarding the question of whether homosexuality is or is not "natural", it depends on the context. At one time I said "unnatural"... I have seen arguments to the contrary that have left me uncertain whether a certain level of non-standard sexual behavior may be more commonplace in nature than I once believed. Now, that in and of itself doesn't directly correlate to morality, norms or law.... there are plenty of things certain animals do that we condemn in humans (including murder, cannibalism and incest). :shrug:
Outside of the context of Christian beliefs, I tend to mind my own business really. For me to tell a homosexual who is not a Christian, that their lifestyle is sinful in my religious tradition is probably futile and will mean little to them.
Socially and legally, I would prefer that marriage remain a male-female thing. I think gay marriage is a misnomer and a contradiction to thousands of years of human society and custom. If my society decides to legalize it, however, I will have little choice but to play along, won't I? And it looks like that will probably happen someday.
Mostly though, I really don't spend that much time thinking about 'teh gayz'.... it just isn't one of my Big Issues.