In my recent fortresses of relatively short lifespan, less than 10 years, my founders always pair off as lovers and I usually get one or two pairs getting married and having children. Just once, a single adult migrant became the lover of one of my founders but something happened to that fortress before they got married. I have not had any pairs of single adult migrants become lovers.
At the beginning of the fortress I have a small meeting zone, about 5x5. When the larger waves of migrants begin to arrive I go to an 11x11 meeting zone. After a couple of years I build a large dining room which doubles as the meeting area. In combination with the data above, this supports the theory that confining Dwarves in small meeting areas encourages marriages. The problem is that some animals will fight if they are allowed to freely go to the meeting area when it is small. It is probably best to pen all of the animals elsewhere, even the pets, if you want to put a large population of Dwarves into a small meeting area. I believe you also get an increased number of grudges, with whatever consequences that entails.
Edit: In a previous version I used to make two different meeting areas, one for singles and one for the rest. I controlled access with burrows. My theory was that forcing singles to only socialize with other singles would hasten marriages. It required a lot of micromanagement as Dwarves arrived, were born, or got married, but it worked well.