She meant marriage as an institution not monogomous heterosexual coupling to be clear. "Marriage as an institution developed from rape as a practice." -  Robin Morgan "We can't destroy the inequities between men and women until we destroy marriage" - Robin Morgan She was always talking about a patriarchal institution that at the time gave women no rights to credit in their own name, abortion, no fault divorce or the right to sue for sexual harassment in the workplace. 
A lot of Robin's early editorials have been lost, but she had a very complex complicated analysis of heterosexual sexual relationships early on similar to Kate Millett (who was married when she wrote most of her work). YouTubers tend to make very complicated theory over simplistic. The idea that Radical Feminism is trying to "destroy the family unit' came directly from the John Birch Soceity and was propagated by Phyllis Shafley.