Sexual identity, or sexual orientation. There is no proof that anyone can change sexual orientation. However, let's say for the sake of argument, you are correct, and one can do so. What if people like the way they are, and don't want to convert to be like you are? Simply because someone can change, doesn't mean that person should change. Does this mean they who choose to not be like you should have fewer rights, less support and less recognition than you have?
There you go putting it on me again, accusing me of bias, when I ask you to prove your own assertion. What do you know about "the main way" gay men engage in sex? Perhaps you are bias. Most, if not all, parts of the body, are used for many different purposes. Why should the anus be different? I can see it now, a man not getting a prostate examination, because he believes he wasn't "designed" to have anything directly put "up" there. He was better "designed" to have his cancer go undetected and untreated.