This phrase irritates me.
True marriage has three components:
1) It is permanent
2) Sexual fidelity is required
3) In some places and at some times, it is a contract between one man and one woman (polygamous marriages have been true marriages in the past).
Of course, the sexual dimorphism of marriage is a given. That's what the word MEANS, for heaven's sake.
Now, as far as I am aware, there is no public movement of any size, anywhere, which advocates banning divorce (or, more properly, banning remarriage), nor is there any movement to recriminalize adultery.
If you don't support those two things, then it's really not correct to call yourself a supporter of true marriage.
Had an older Preacher at a country Church speaking on the subject of homosexuality some years ago . He said he was tired of the candy coating going on. “Back in my day”, he said, “we called them ****suckers”. From the Pulpit.