I was like your children, OP. I had a happy home life, was content in myself, had friends to socialise with. I didn't feel the NEED to seek out a boyfriend. I did have a couple of friends who had more difficult family lives and they never seemed to be without a boyfriend, I remember thinking that it was almost as if they were seeking something they didn't get at home. If they are happy and content as they are, and not lacking in confidence, or being very clingy with you, then you are doing well as a parent, and relationships will come naturally when the right person comes along.
At university I started socialising more and found that boys were interested in me. I had a few "dalliances" but really was still happy on my own and not looking for anything serious. Then at 22 I met someone I just clicked with and we went out for a few drinks. Long story cut short, we've been together 26 years and our eldest has just completed his first year at uni. I still say that if I hadn't met him I maybe would have stayed single because I was honestly happy on my own and wouldn't have wanted to settle for just anyone as a boyfriend.
Our eldest is 19, had no experience (as far as i know) with girls, is quite nerdy, and reserved and not too socially confident. To my surprise, he met someone at uni a few months ago, they clicked, and have been going out ever since. He's met her family and she's staying with us soon, so they seem very keen on each other.
Our youngest is in 6th form and is all about his mates. Never had a girlfriend, shows no interest in girls. I am not at all worried about him, relationship-wise. It will happen when it's the right time/person. As long as he's happy that's all I'm bothered about.
I understand what you mean in some ways. YOU know that your children have a lot to offer, and you'd like them to have a special person who is on their wavelength, on their side, who just "gets" them, and who they feel secure with. But there really is no rush. It'll happen when it happens. These days you don't even have to go to bars/clubs to meet someone. My nephew met his girlfriend online gaming at uni, met up in person and clicked and have been going out for months.
Please don't push it, or show your disappointment, or worry. There really is no need.