If these extreme "introverts" won't talk to anyone yet are married or have a partner, how on earth did they meet their partners?
Through their workplace, hobbies, etc. Most aren't "extreme" and will engage better with people in smaller groups. There's a massive difference between not wanting to get pissed at parties or pubs every weekend or sitting around gossiping about slebs, as opposed to joining clubs etc which are usually a lot smaller/quieter with people having likeminded interests.
Personally, I've always preferred working in small firms - my only experience working in a large organisation didn't end well! I've always done very well working with a very small group, making some very good friends in the workplace (but never outside).
I met my OH through our local advanced motorist group. It was ideal for us both because it was always one-to-one. Even their social events were usually 1-2-1, such as treasure hunts, manoevrability competitions, table top rallies etc and monthly meetings were usually guest speakers, so if you just wanted to stand/sit in a corner and not be social, that was absolutely fine, but you could also wander around and talk to others if you wanted.
He was also a member of a model railway club. Again, ideal for the less-social, as you could go and just do your own thing or you could talk to people - entirely up to you.
The benefit of both was that there was never any pressure put upon you to confirm to the extroverts' idea of normality. No-one thought any worse of you if you did just sit/stand in a corner - that kind of thing was pretty normal, but at the same time, those same people would come out of themselves to engage, often in great detail, about particular subjects that were of interest to them.
It's the pressure to comply that gets me. When you start putting pressure on me, my default is to retract back into my comfort zone. When I feel more relaxed and less under scrutiny, then I'm more likely to come out of my shell.