It's not the mpreg-as-kink-fic that gets me, or even the plausibility issue, really, it's just a sort of visceral "human anatomy does't work that way" reaction. It can be awesomely in character and even logically worked out, but as long as the pregnant character has boy parts instead of girl parts, I run smack into the brick wall of "No, no, he's a guy. Only girls can get pregnant."

The only place I was ever actually sold on it was Ranma 1/2 fic, and in that case it wasn't really mpreg, because Ranma was a girl the entire time. It was more "normal female pregnancy with gender issues."

I'll probably break down and read it eventually, though. It will sit there, tempting me with h/c content, while the back of my brain pokes at me with "both Tony passes out a lot and Steve is all worried and maybe you can just pretend real hard that it's genderswap." My will is weak.