Just because there is no god and no list of rules to follow on a stone tablet somewhere, it still hurts to touch my hand to a hot stove. I still have empathy for fellow creatures and don't want them touching hot stoves unnecessarily, even if it means more food for me. Ants can can still work together for their common good. Monkeys can still hang out in groups for mutual protection and share food. I can still conclude that the immediate gratification of rape & violence pales in comparison to what we can accomplish together in a society if we live peacefully don't rape and kill each other, and we only realize the true benefit of that if we band together and afford protection to all so that no one is raped or killed and they are able to own property and put it to use and have self-determination and liberty and the ability to pursue happiness within reason.
It gets a little trickier when we disagree on how best to provide for the common good and encourage freedom while protecting against exploitation and negative externalities but the mechanism for contemplating these things exists. I don't need transcendental concepts of good & evil to engage in that reasoning, I can make a distinction between good and bad subjectively based on experience, logic, and reason.
And I can tell you that the way the church views homosexuality is antithetical to my understanding of what is good for humanity and I'm opposed to that aspect of their teaching. It has more in common with the harmful and disgraceful aspects of our human history than it does with anything positive or righteous and for the good of all of us I encourage people to come to that same realization with me and let people who find comfort in same sex relationships have the same benefits and privileges that we do because I think it will make the world a little better.
(n.b. I've been listening to the Harmontown podcast a lot lately and that really needs to be read in the voice of Dan Harmon going on a tangent about how we're all primates, etc. because that's how it sounded in my head.)