A driving force in most people's lives is Progression. That is, they want to move along a predefined line in their lives towards a goal. Culture pushes us forward, assuring us that as long as we are moving along the path of School, Acquire Career, Get Married, Have Children, Get Promoted, Retire, Die we are doing well. People who have no interest in following that path are looked upon as being selfish, lacking in goals, lazy, or foolish. I read an interview with someone that could be described as a 'singles evangelist' who talked about how people view those who choose to be single very negatively. There is an assumption that anyone who is single must be damaged goods or deranged because otherwise they would find somebody and get busy with Progression.
The problem is not getting married and it is not having children. I got married and had a kid and that is all fine by me. Progression is a bad thing because it carries the assumption that everyone *must* do these things and that those who do not are doing it wrong. The world has plenty of kids and we really don't need to increase the rate of production. When you take away children as a necessity you also reduce the need for marriage tremendously - why worry about being married or not if you aren't trying to create a stable, long term environment for children? Of course children can be raised in all kinds of circumstances but one cannot argue with the fact that having multiple parents around long term is very useful in the raising of kids.
Many video games actually have this exact same debate going on but framed in different terms. In WOW, for example, characters are constantly striving to acquire better and better equipment. There is a huge amount of pressure to maximize that Progression and push towards being 'perfect'. There is also a tremendous amount of vitriol directed at those who choose not to take part in Progression and who simply mess around doing whatever is amusing at the time. It is clear that we as a species have some innate drive to push towards goals and also to want others to have the same goals. We have a desperate need to feel that we are Progressing and moving ever onward and upward.
I think the fundamental issue is that if we let everyone define their own goals the goals we have chosen for ourselves suddenly feel less universal and necessary. People often want to just get high and have sex for their entire lives but they go out and do things because society tells them they need to Progress. Once they are on board with that plan they usually feel compelled to push Progression as the only way - after all, if it isn't the only way then why not just go spend every day napping under a shady tree? I tend to advocate for playing video games instead of shady trees but the principle is the same; do what makes you feel good and don't worry about Progression. It is a sucker's game and the only way to win is to not play.