On the most basic level, I have never in my life been attracted to the shape of a fully pregnant woman. I have to assume it’s coded in somehow, and why that is is what the other arguments are attempting to rationalize.
I wasn’t traumatized by any pregnant women or anything, I can’t think of a reason it would be “nurture” as opposed to “nature.”
Regarding maintaining a pair bond through periodic abstinence, how do you make it through the time immediately afterwards where there are serious physical reasons why doctors tell you they need to heal? Do you have to do it anyways and risk serious harm to push the emotional aspect along, or can you just survive a couple months by affection and letting her pick a romcom to watch when she wants to?