Before I had a baby only one thing could make me gag to the point of throwing up. Not poop. Any pet owner gets over fear of poop right quick, and we have two cats. Not puke. Puke used to do it, but then I became a borderline alcoholic and the smell of vomit, although not something I sought out, didn't really bother me that much. No, it was boogers. Well, boogers and spit. Anything that qualifies as mucus.
I think something happens when you have children. Your grossout switch gets disconnected or something. Dash has literally been covered in boogers and drool for FOUR FUCKING WEEKS and last night I scooped some out of his nose with my pinky nail (after the aspirator failed again) and, when I couldn't find a rag nearby, wiped them on my pajama pants. And continued to wear them for the rest of the night. And then hung them up on the hook in the bathroom to wear again tonight. Pants that are covered in my son's snot, you understand.
So I guess it's two switches: grossout & dignity.