There is absolutely no reason to believe that restricting your data to shotgun weddings would control for the things that they claimed to be controlling for.
1. The man may be more likely to acquiesce to the wedding if they have a stable income and can support a family.
2. The interested parties on the bride's side may be more interested in pursuing a shotgun wedding if the man has a stable income.
3. The same reasons that lead women to pursue marriage with high-income men may also lead them to pursue pre-marital unprotected sex with those men.
I'm sure you can think of more. This seems like a very suspect way to segment a population.