You are WRONG. Nature "invented" marriage when it demanded that a man has to have sex with a women for procreation.
Obvious nonsense. Nature is about reproduction. How that is done is reproductively irrelevant. Rape passes genetic material. Sex with reproductively viable children passes genetic material. I wouldn't consider either to be particularly 'moral', nor the 'invention of nature'. But they serve your 'nature's plan', don't they?
As do polygamy, one night stands, harems, orgies, gang bangs, etc..
If marriage were intrinsic to nature, then anything that reproduces would be married. Yet only we are.....because its our invention. A social construct we made up for our own convenience. And it is whatever we say it is.
Our construct isn't particularly connected to reproduction. The infertile can marry. The old can marry. The childless get all the same benefits of marriage as those with children. No one is required to have children or be able to have them in order to get married.
Why then would we exclude gays from marriage based on their inability to meet a standard that doesn't exist and applies to no one?
The standard exists, and always did. Since nature designed us that we need a male and a female for reproduction, that coupling was called marriage because human offspring need both parents to develop in a well rounded manner.
In essence, humans named reproduction "marriage" to lead to a stable family unit and society.
So, the standard does exist, and applies to everyone.
Mark