Uhm, animals don't have moral law and are required by God to marry before procreating or attempting to procreate. So that's irrelevant.
Now, what they're implying with this line of argument is that sodomy is NOT contra naturam because, see, you can find it in nature. That's not really what that term means. What it means is that you can infer God's intention for nature ... whether or not all of nature at all times abides by those intentions. In other words, you can infer from nature that the reproductive organs were designed to reproduce and that they were frustrated in that intended design when they're used in ways that are incompatible with that end. Some people are born with birth defects, and as someone else put it, much disorder entered the natural world after the Fall ... but it doesn't mean that a birth defect was the order or the design intended by God ... just a case where something went wrong.