What would you reply to this?

The decline in marriage is mostly to the increase of independent women and feminism. Many women had to marry into families who would be able to financially support them and live comfortably. Women were responsible for having children, maintaining the home, and serving her husband. Some women had to settle for rich, but it came with abuse and infidelity. As feminism and women's independence increased, it allowed women to financially support themselves and not settle for minimal for which they can do themselves. Women were allowed to work and get a higher education.

Another reason is finance. Many women and men live together but do not chose to get married. Combining finances can become a burden to one party. Having to financially support another person can cause a huge strain on a relationship. Before women's independence, the goal and dream was to get married. Now, goals are to travel, be financially stable, mentally stable, healthy and happy and marriage does not offer that.