system
A chat on subreddit /r/NoStupidQuestions
human
I tried that once with volleyball but nobody else joined alone or cared to talk to anyone other than the person they brought QQ
gpt
I've heard that competitive team sports are the exception to this rule - people are there to work out and *win*. They can also be cliquey. That doesn't always lend itself well to romance. 

I hear rock climbing is great for meeting people because it's more cooperative, and for certain activities you need to have a partner.