What does it mean to support marriage?
The most important rule of marriage is the forbidding and punishing of extramarital sex since the purpose of doing so is to make couples marry in order to have the right to have sex and children with each other.
If you allow people to have sex outside marriage ie fornication, sodomy, adultery etc, then people would predictably use these marriage substitutes to have exclusively recreational sex all their lives and not be troubled by the expense and inconvenience of rearing of the next generation either properly or at all.
A declining birth rate is a sign of a dying civilisation that cannot even replace itself in sufficient numbers to fight a successful defensive war. This means that such a society would have the characteristic of a risk-averse elderly woman and nations with this characteristic will be easily conquered by nations with the characteristic of enterprising young fighting men.
Slut single mothers, men who have sired bastard offspring with slut single mothers, bastards and sodomites who acknowledge that the next generation would be better off brought up by two married parents living together would at least support marriage in principle if not in practice, if they cared about the national interest.
If slut single mothers, fathers of bastards and LGBTs can neither acknowledge this truth nor accept the imperative of promoting the national interest, then they are clearly doubly subversive, making it even more justifiable to ignore their anti-marriage views. The views of those who are both selfish and intellectually dishonest should certainly be ignored.