"True Marriage" are words, nothing more. The covenant of marriage is something that was perpetuated by the church for a few thousand years to ensure procreation to continue indoctrinating more people back into the church.
The "equal rights" of marriage are afforded by the government; tax breaks, health care regulation, dying wishes/estate care, etc. Anyone who is in love and making a commitment to another person, should be afforded all of the same 'rights' regardless of the gender of their loved one.
I also like how your whole "it isn't natural, people would die out because we're going against natural selection'' argument fell flat on its face - so now the words "true marriage" somehow mean heterosexual. They're words, literally, that's it.