>>813642
The biggest issue with this is that if you start from a position of irreligious individualism then marriage itself becomes meaningless, and redefining it is just a step to undefining it, for it is a byproduct of a society that was both religious and communalistic. You need one of these for marriage to make sense. From a communalistic point of view, marriage encourages the family unit, which is the smallest self regulating unit after the individual (who are usually very bad at self regulation). The family unit also allows the continuation and expansion of the society, through the procreation of children.
Now, the opponent might ask, "what about those who are barren and cannot have children? Why should they marry?". And the answer is that a barren man and woman who marry can make up society's image of the family unit. A communalistic point of view suggests that the community has an ideal way of life whose main purpose is to continue the community. Everyone has faults, whether physical or mental, but we all strive to reach the communal ideal.
And this is where we hit a roadblock: the above makes absolutely no sense in an individualistic society: what does the individual care about the community, beyond his own satisfaction? So if he has perverse desires, he will indulge them and encourage others to do so, breaking down the communal ideal.
There is a whole other topic about how homosexuality can spread - specifically how perverse desires can be inflicted upon young children. Two recent examples were Milo Yiannopolous and George Takei confirming they were molested as young teenagers. Before backtracking, Milo had suggested that man-child relationships can help a child "discover who they are and give them security and safety and provide them with love…". He later confirmed he had been molested as a young boy.
The communal ideal eventually can be threatened not just among homosexuals, but heterosexuals who desire that lifestyle. Much of the casual sex culture in western society was predated by gay culture. Casual hookups and drugs at festivals? They gays don't have a monopoly anymore…
But as said, you will hit a roadblock because unless they are either religious or communal, they will reject this argument completely. Their headspace does not cross paths with ours.