Reproduction is triggered when two blobs of opposite gender who are fully fed collide, and even then it doesn't always. The female has to be the one to collide with the male.
There is an issue where blobs will randomly assign you as an "enemy" for some reason, and in early game that just means they run away. Not sure what's causing that, that's why I had two starting mates so if one runs away, you have the other.