Truly, that passage was not well explained. It's a complicated issue; let me try to round up a few ideas:
1) After same-sex marriage was legalized in the Netherlands, the rate of heterosexual marriage declined markedly. Marriage and particularly natural parenthood had been "officially" redefined, devalued, and desexualized.
2) The pernicious sweep of "no-fault" divorce from the 70s-80s onward "officially" removed moral considerations from marriage and parenthood. This legal fad more than perhaps any other has contributed to the gay lobby's belief that marriage is a "right" without the conventional restrictions that gays do not practice to the same extent that heterosexuals do, such as the expectation of faithfulness to the one person to whom one is married.
3) When divorce between the biological parents of children became more commonplace, accompanied by a rise in petitions for government to allow adoptions by single persons and eventually by gay and lesbian persons, an industry sprang up to "help" all these persons become divorced from biology — attorneys, counselors, divorce accountants, therapists, educational consultants, authors — many leftists seized these opportunities and, like the Global Warmists, do not want to lose their lucrative hustles.
4) The divorce industry creates "winners" and "losers" in inverse or in perverse relationship to the care of children. If one parent gave up working to care for the children in marriage, the other parent is seen as more financially capable of caring for the children at divorce. If one parent tried to impart traditional marital values such as fidelity or religious observance within the marriage, that parent would run the risk of losing custody at divorce, being characterized as "bigoted", "hateful" or "discriminatory."
5) Awards of child support for anyone but the very wealthy are typicallly substantially less than half of the actual cost of raising the child; yet, with the cost of now running two households, they are onerous also to the one paying the support — a lose-lose proposition frightening many from the prospect of marrying at all, or bringing a child into the world if pregnant.
When marriage is legally divorced from the onset of sexual activity and from morality itself, and when marriage and childbearing are legally divorced from fidelity, economic support for caretaking of children and from permanence, society experiences a lowering of expectations and morale, and even a desperation at the perceived lack of opportunity to live in a loving family with children. It is no surprise that substance abuse would accompany these conditions, as it did in the Soviet Union after their abolition of religion; however, I agree with you that the cause-and-effect cannot be perfectly laid at the door of the onset of gay marriage legalization alone.
Legalizing ssm has opened the doors to other forms of deviant unions. It should come as no surprise as it drags us back to a pagan lifestyle. In case you missed it,
Teen plans to marry father, have children
It's all about love!