Just so you know: I just confirmed a theory I had.

If you want your single peep to adopt a child before marriage, you just have get a peep that "definitely" wants to have a baby and let he/she stay single for a couple of years.

That happened 3 times for me.

My fourth try (for the sake of the theory) was to start a family with a "not really" wanting a baby peep. She stayed about 10 years single and with no adoptions.