As an unmarried Young Adult (in the Netherlands) myself I can give my 2 cents about why I wouldn't get married; I've simply taken to heart the advice nearly every senior developer would give me; Don't EVER get married.
That is very weird advice. I've been a software developer since the mid-1990s, when I was a teenager, got married at 21, had two kids, started several companies, sold some of them, and getting married is easily the single best decision I ever made.
I would like to gingerly suggest that this is a subject for which you might not want to sole-source your advice from software developers. We are an odd lot.
Perhaps software developers in the Netherlands have geographically-clustered attitudes about marriage that more closely resemble those of Dutch adults generally (or maybe Dutch academics or trades-workers specifically), than software developers generally.
This is actually a topic that has received a lot of investigation. It's true that men who marry differ, before marriage, from men who don't. But it also seems to be true that marriage has a strong causal effect on male behavior.
There are a lot of instances where long married men lose their wives then die very soon afterwards. When there's a correlation of something with longevity and removing the something is soon followed by death, then there's good reason to believe the causation theory.
True for both partners -- usually after one dies the elderly widow/widower tend to have ~5 years or less.
Some of that is just timing, like if the average age of death per the actuary tables is like 79, and your spouse dies at 80, you're statistically going fairly soon as well.
As far as I recall, there were studies showing some causal effect in the behavioral difference of how early things like cancer were diagnosed.
In essence, the average man living with a female partner would get a "nuisance" investigated, diagnosed with cancer, and (sometimes) treated; while the average single man sought treatment only when the condition was truly disturbing, and by that time it's too late to treat the cancer.
Keep in mind that unhappily married or divorced people are the ones who are more likely to say something about it. I'm happily married. I don't really mention it, and I'm sure others in a similar situation do the same.
I would like to gingerly suggest that this is a subject for which you might not want to sole-source your advice from software developers. We are an odd lot.