I don't think we're being deliberately vague, the answer just varies for different people.

Generically, bad places to pull a spouse from: online dating sites/apps, bars, concerts, large parties, etc. Just about anywhere that attracts people from a variety of social strata and offers zero curation for common values, education or habits of mind, and/or doesn't sort people in any way based on useful proxies for those things.

The trouble is that those sources are tempting if you are socially isolated (from the opposite gender) and perceive yourself to have few options, and/or because you're lazy or fatigued from the ever-frustrating chase.

Always bad? No. But the median outcome is going to be worse than with other, more enlightened dating strategies.

So, lonely guys in male-dominated jobs get desperate and make bad dating choices in an attempt to avoid loneliness, which often ultimately backfire.

Yup, that's about what I thought.

Yeah, but when most people think "backfire", they think "painful breakup" backfire. It can get a whole lot worse than that. "Bad dating choices" can be an underwhelming euphemism at times. It all depends on what kind of hornet's nest you get into.

If you marry the wrong person because you dated the wrong person via one of these mechanisms, you can end up in a really dark place.

Right (been there). Any advice on solutions? What are the good places, how to join?

If I had a good answer for you, my own life might have gone differently.