Let's just see what natural law he might have had in mind. To begin with, the whole idea that something can be 'natural' or 'unnatural' is not even a Christian concept. It is a concept of Greek and Roman philosophers like Seneca. Talk to the animals Following Peter's suggestion, we should we look to animals for guidance and example in sexual matters.
Animals are by nature promiscuous.
They do not care about marriage or fidelity. They do not care whether or not another male or female is perhaps a parent or mature offspring. Perhaps he finds gay sexuality to be reprehensible because it is unknown among the animals. It is well known that there are few species of animals among which same-sex activity has NOT been observed. In fact, most animals in the mood will go after anything even close.