Nappies are no big deal but I honestly think it's simply some switch in the parents' brains that goes "let's not worry too much about this, it needs doing". You filter out the gross and you simply do it on autopilot.
I sometimes wonder about the people who must clean messy public restrooms. All of the gross, none of the "but it's for the sake of a cute human that I love".
I used to clean bathrooms in a Manhattan shopping mall. I now have a tiny human. Cleaning the bathrooms was much, much more disgusting. It has been nearly 15 years and I still think about the horrible things I witnessed at that job. Changing my daughter’s diaper doesn’t even register to me as a gross thing to deal with.