Through gay marriage, marriage itself is going to be very loosely defined. Gays often don't have monogamous relationships but have "open marriages", if they get "married", which of course, from a traditional view of marriage, defeats the purpose of getting married at all.
The purpose of marriage is to provide a structure in which a man and a woman are in an exclusive relationship (in historical times, there may have been more than one woman, but I believe that one man and one woman is ideal) where they complement one another. The man provides love, protection, financial support, and sexual satisfaction to the woman with the woman providing love, emotional support and praise, sexual satisfaction to the man, and bears children for him. The main commonalities in this are love and commitment to one another and to their common life and goals.
An open marriage or homosexual marriage redefines and defeats the purpose of marriage. I really don't know why leftists believe in marriage at all.