If you look at the details both Dash and Molly have blue eyes, blond hair, and triangler ears with rounded out edges, and when they smile it looks like there are missing teeth. Plus with Dash's super speed he could have easily escaped a relationship. Also, Dash has super speed so if he wasn't ready too father a child, he could have easily just run away. Dash is also not likely to stick around, explaining why in none of the Toy Story movies, you see Andy or Molly's dad.
Theory by Jacob Thore