Why is being gay a sin?

I don't understand, having sex with a girl isn't much different when you look at the clitoris and it's structures. http://serendip.brynmawr.edu/biology/b103/f05/web1/na.html Why would God give the female counter-part a male genitalia part that resembles it and when you look at the spotted hyena or a hyena it has balls and testicles or a resemblence so how is being gay a sin if a female has a female penis (clitoris) and how is women having sex with women a sin, I do it with prostitutes in Las Vegas some of the time, we aren't that much different then males other then we have to shave our legs and arm pits, i don't see it. I think I am an atheist, what I found out about the clitoris is disturbing and gay sex can be seen as similiar; a spotted hyena sticking it's wang into a spotted hyena's females peusido-penis.