Being attracted towards the people of the same sex is not a sin in Islam. However, homosexual acts (sodomy) are considered as grave sins. Allah almighty, may he be exalted, says regarding sodomy (interpretation of the meaning) :
[7:80-81] And [We had sent] Lot when he said to his people, "Do you
commit such immorality as no one has preceded you with from among the
worlds? (80) Indeed, you approach men with desire, instead of women.
Rather, you are a transgressing people."(81)
[26:165-166] Do you approach males among the worlds. (165) And leave what
your Lord has created for you as mates? But you are a people
[29:28] And [mention] Lot, when he said to his people, "Indeed, you commit >such immorality as no one has preceded you with from among the worlds.
[27:55] Do you indeed approach men with desire
instead of women? Rather, you are a people behaving ignorantly.
Its a grave sin because it goes against the Qur'an and Sunnah. It's also against our fitra (natural disposition of mankind).
Some might argue that homosexual behaviour is found in many animals, so its natural. My response to that is, if animals sodomise each other, that doesn't mean its natural because animals don't have intelligence. Animals can't choose between good and evil but we humans can. Sodomites aren't born sodomites, they are sodomites because they engage in sodomy of their own free will.
There are similar activities (eg. necrophilia) that exist in animals but that does not mean they are normal. Prohibiting necrophiliac behaviour (sex with dead bodies) and permitting sodomy is plain hypocrisy.