When do people decide they want to be monogamous? Do they choose, or are they pressured (mostly wondering about women)

Why do some people want to stay in a long term relationships at age 18, 19, 20 etc? I understand not everyone wants to “sow their oats”, but are some of these young adults expecting marriage and a happy family that will last from age 21 until they die?

Do most people that enter a long term relationship expect it to turn out favorably regardless of age (for simplicity sake, a “healthy family”)?

Is the method of sowing oats a conscious thing? Do some people say “I need to get a variety so I can know who I want to settle down with”, or is it just something that happens until your biological clock and experience change and you decide you want a long term partner and a family?

I think in general, I’m wondering: are we consciously looking for a mate for monogamy by going through partners? Do we expect some relationships to eventually fail? We can’t expect the first relationship to be the last one, of course. And how does age play into this?