Same sex penetration among non-humans is indeed rare, but direct genital-genital contact is quite common. I don't know about you, but I figure if I were rubbing my erection against that of another guy, that counts as sex.
However, I feel that stressing that other animals do this, and therefore it's natural is a bad way to go on advocating for our rights and acceptance. Things which are natural are not necessarily good (botulism), things which are unnatural are not necessarily bad (this website). Arguing morality from naturalness is a common mistake--referred to as the Naturalistic Fallacy, actually--and in our case I worry it might cause people to start viewing homosexuality as a disease again. Personally, I prefer the "What harm are we doing anyone with our private behavior among consenting adults" track, but that's probably due to my libertarian leanings.