If leviticus is part of the old testament then why do SOME christians use it to condemn gay people?

It seems to me that if it is old law then you have to disregard it. There are a ton of silly rules in that book of the bible. If you have to disregard it then why not disregard the tidbit about gays. Is there a book in the new testament that says it's wrong? If there is why not just use that?