Ignore the nasty posts. Some people get their kicks from bullying others when they are vulnerable - and they are accusing someone they have never met of possibly being not very nice. Irony check!
I'd be worried too. Your reaction is normal and natural. Any mother would care about that key aspect of an adult child's life.
I know two men who are in similar situations. One is same age as your son. He is a gorgeous young man - handsome, good job, but naturally quite shy I think, especially around women. It's a mystery to me why he is single.
The other is drop dead gorgeous. The kind of looks that whenever he walks in a room women and gay men ask me how I know him and will I introduce them. He is nearly 40 now. He's had no problem finding hookups and ONS but doesn't seem to know how to build that into a permanent relationship. I know he is lonely now but he got into the habit of casual relationships and I think doesn;t know how to make the shift.
Have you had an honest chat with him about it? Have you asked if it concerns him? If it doesn't, you may just need to back off. But if it does, you could ask what he feels the barriers are and help him overcome them. I did this for DS2 who was painfully shy and socially awkward - not helped by starting uni during the pandemic. He started to write more specific profiles on his OLD saying exactly who he was faults and all, and what he was looking for. That really changed things for him and he has since had good luck.
I was also quite blunt with him and chatted a bit about what women did and didn't find attractive. Confidence matters - so he started to build it. And I pointed out one of his hobbies is conventionally seen as sexy and he needed to get out there and do it more. He did, and met a lovely girl that way.
Does your son have any hobbies that are attractive? You say he runs. Can he join a running club rather than go running alone? A friend of mine met his girlfriend that way. Is there a way you can gently encourage him to do things slightly differently (so long as he agrees that he wants to start dating and wants advice)?