There are some people who think that homosexuality is a choice. They would argue that we can make this choice because we have free will. I will focus on those who think this way because of religious reasons. Those who think that God gave us the freedom of choosing right or wrong. But God is also the creator of the animals. So why do most of the animals manifest homosexual behaviour? They do not have free will. They cannot chose, they do not know what right or wrong is, they just follow their insticts. Why would a God who thinks homosexuality is a sin, create animals who have homosexual relationships?
Here is a list of mammals displaying homosexual behavior and here is the link to the wikipedia article about homosexual behavior in animals.