Hmmmmm. OK…We could start with the Adam & Eve experiment. You know, Man + Woman = Children. But I like to stay away from the religious aspect.
Humans have three natural functions. We eat, sleep and procreate. The only physical or biological difference is that of sex. Male and female. Again that difference is related to procreation. It is perfectly normal and natural for man and woman to join together for the purpose of procreation because of their natural design. This is innate. Man and women are born that way.
Another form of proof that should appeal to you is the Home Depot Analogy.
In the plumbing dept you can buy a “Male” pipe fitting and you can buy a “Female” fitting. No matter how much Teflon tape or pipe-dope you use, you cannot connect two “male” fittings together.
Same with the “Female” fittings. Interesting that when you join a “Male” fitting with a “Female” fitting it creates what is known as a “UNION”. Anything else is abnormal.
One more form of proof is you. That’s right, Oldcatholicguy….you.
You are here ONLY because heterosexuality (normal, natural sexual relations) is innate.