Climbing - you already go to gym so are in above-average shape. Its a sport for 2 usually. In pre-covid (and pre-children) times I used to organize 1x per week a climbing session on our local meetup-like web. Usually 4-6 people showed up, often new ones.

Eventually you will meet interesting people. Even some girls who may have similar mindset to yours. I've met more (former) girlfriends through this than any other means, and eventually my wife.

I consider it the best sport out there due to mental aspect of it, especially on rock (and ski touring in winter). But if thats really not your cup of tea, look for other sports. Team ones, or some course where you end up with bunch of others. Just expose yourself to the crowds, even if its not that pleasant for ie introverts (like me).

Doing above, I've regularly ended up with more close people than I could possibly manage to keep friendship with. Also non-trivial number of ladies were keen to meet for other adventures like hiking, ski touring etc. Tons of people are wanting to do these sports, but they need some buddies.

I started playing tennis again, building on some basics I learned as a kid. Joined a local club, and was dumbfounded why no one was playing on beautiful weekend summer days. The sport has declined a little but still, where is eveyone?

Turns out all the action is at the organized league play with teams of 6, playing at home or travelling to nearby clubs. Through age groups and play modus anyone who can nominally complete a match skill-and fitness wise can join and contribute both to team success and social life surrounding the play. Through the shared activity you can connect, and meet for a match before joining people for non-tennis activities, before "making a friend as an adult".

So yes, the social structures are eroding rapidly and this will have major consequences for society, but there are still pockets left that you'll have to go out and find.

my city has multiple recreational sport leagues that are active, and I spend a good chunk of time volunteering and have met some great people that way.