You can make the argument that adultery is bad because one party is being hurt- the spouse being cheated on.
Your argument against homosexuality is that you think it's icky. If both partners are into it, and that is their sexual orientation (not a choice), then it isn't "sinful". It's just the way they turned out, whether you want to call it "God" of "nature" or "fate".
What special right is that, that they not be discriminated against based on their sexual orientation.
What if a bakery decided to deny service to an interracial couple because that was his interpretation of the Bible? Would that be okay?