I'm plenty happy living my life as I see fit, and I don't see marriage as a potential addition to that happiness. It's as simple as that.
Then why would you imagine parents tell their kids that life is "fucking horrible", instead of telling them that life is "plenty happy"? Like, everything you describe, you have or will go through as well, and apparently you don't find it to be "shit. Absolute shit".
One lap around the course is plenty. If others want to bring more horses in, they're welcome to; as for me I'm not interested.
When did I ever say that? I don't care about others' lives, it's none of my business.
I find life, as far as I experience it, to be overall horrible. I'm plenty happy regardless, but it's nonetheless not something I'm interested in foisting onto my hypothetical kids.
Nor is anyone asking you to. Just, don't go around assuming that parents tell their kids that "life is, on average, fucking horrible"—instead, they show their kids how to be happy, like you are.
You're leaving out all of the joy, the triumph, the beauty, and the awe of having a brief moment of consciousness on this planet, in this universe.