Turns out, Mom's question is a big deal. Tristram explains that the question interrupted his dad (doing what?), which scattered and dispersed the animal spirits (1.2.1) that were supposed to lead the homunculus to his destination.
Homuncu-what? That's the idea that sperm already contains a future person, minus a soul (little Tristram).
Tristram explains that the coitus interruptus meant that his little fellow (my little gentleman) had to make his way to his destination all alone, and arrived worn out, ruining his life forever (1.2.3).