Because denying someone quality housing based on their sexual preference is vile and reprehensible. You aren't "contributing to sodomy" by treating people fairly. You contribute to making this world a better place. It's not up to you to judge.
You just contradicted yourself. If someone truly finds homosexuality disgusting why should your morality fore them to rent to a same sex couple? You are saying your definition of vile is superior to theirs.
I would agree with you if you are talking about a government job or housing since gays pay taxes as well. But you didn't buy the property so you shouldn't decide if a guy wants to rent to a young black heterosexual couple instead of two white lesbians.