I’ll try, although it’s been awhile since I studied natural law.
Natural Law has to do with the nature of things, and the nature of people–how they are made, and what they are made for.
Because men and women are designed to be physically complementary, we can infer that this complementarity is for a purpose. That purpose is so that the human beings can be fruitful and multiply. Physical complementarity brings about the next generation. That’s just the nature of the sexual act.
Natural sexual relations always have this possibility of bringing new life into the world. And that new being will require care and nurturing for 18 to 21 years or more! That means that the act itself means bonding and permanence. That’s why it should only occur within a committed, lifelong relationship–in other words–marriage.
Deviant sexual relations, of whatever type, do not fulfill this natural purpose, this natural complementarity, and are thus, even at the physical level, un-natural.