Perhaps consider Teleology in the sexual realm? Teleology is the study of purposes of things. What is the end or purpose of sex? If you look at what bodies are designed for and follow that you'd come up with a fairly conservative outlook on sexual norms.
For instance, all the biological systems (respiratory, digestive, neurological, circulatory) are complete within a single human body EXCEPT the sexual reproductive system. That is incomplete and requires another body- but not just any other body; but a complimentary body. Therefore, you can argue that from a purpose driven standpoint, sex is designed to between a man and a woman.
Also, what is the purpose of that system? The fullest result of the completion of the system is a baby; hence you could argue that the only correct, non-deviant sexual act is that which has the possibility of children. 759
(That doesn't mean that every act must have that as its goal, but rather you have to have the two pieces of the reproductive system complimenting and completing one another.
While one could argue that sex can be used for other things (pleasure, bonding), even those other aims are actually parts of the reproductive system that are geared towards having children: The pleasure to entice us to engage in something that could require great sacrifice; and the bonding to mold two people into a formidable team that can raise the young.
Here you could suggest monogamy as the norm by the bonding and raising of children.
Since science has yet to find any other genders; and we know that those who watch porn actually alter their brains (neuroplasticity) so that they come to want, and desire, to act out what they watch; you could use those ideas to argue that they are part of the source of deviancy.
This would be possible to get to by looking at the natural law and the natural world. Instances in nature that suggest other deviant acts as ok may actually just be instances of perversity or depravity in nature. the 'appeal to nature' is actually a fallacy in philosophy; but you can look at the order that nature has set up.
Note* You may have to enshrine this idea of not going against the design and nature of something into your culture as atheism may have a tendency to believe there is no good or bad and thus there is a temptation to tinker.. You'll have to find a way to get around that. 140