Uh, NO!! That's not true. The anus is for expelling waste. Over time, anything rammed up there damages and weakens the muscle function. Homosexuality is certainly a mental disorder. 
Actually, yes, that is true. The anus contains the male g-spot. The penis is shaped perfectly to penetrate an ass, to get to the g-spot. Explain how that's not 'a function of a penis', then?
You know nothing about anal sex.
Prove that it is a mental disorder. I fail to see how one sexuality could be a mental positive, and the other a mental negative. The only difference is what is in from of 'sexuality'.
-------
*Forever & Always...Yours*