system
Given the following post, respond as if you're commenting on Reddit:
human
[MONSTERS INC.] How do the monsters breed? Are they different species altogether, or variants of the same species, like dogs? Do they have to get with someone of the same 'type'?

In Monsters University, Squishy and his mother are both the same 'variant' of monster, and we presume the father is too, leading us to believe only two monsters of the same 'type' can breed. What if Mike and Celia were to have children? Would all children look the same? Would they be a mix of the two parents and create a new 'type'? 

Secondly, do the monsters even have sexes? They're referred to with at least two pronouns ('he' and 'she'), but as far as I'm aware have no defining sexual characteristics.
gpt
Potentially like Pokemon.  In Pokemon, creatures of different species are capable of breeding as long as they're within the same "egg group".  Monsters seem like they belong to one "species" - monster, but because of differences in their morphology it's likely that there are certain groups that are physically capable of reproducing and having sex with each other.

Ex. Mike and Celia are most likely capable of having sex with each other since they're romantically entwined.  

It's not definite that all can have sex with each other though, and with such wildly different bodies, it's unlikely that say Sully and Roz could fuck.

The babies would probably take after both parents. Mike and Celia's baby would be one-eyed, maybe have horns, maybe have snakes, could be tall, could be short, maybe pink, maybe green.