You just said, without apparent irony, "How come you didn't know you didn't enjoy this experience until you had this experience."
Just to make it abundantly clear: looking after someone else's kids isn't parenting.
I don't think there IS a way to experience pregnancy without getting pregnant. A sandbag around your middle doesn't tell you what your hormones will do, before, during and after. And so on, and so on. And, being a bloke, not having the same day-to-day hormonal exprience to begin with makes that level of understanding nigh-on impossible.
On a more general point, when we think about things we can juggle around seven ideas or concepts at a time, when we experience things we are processing dozens (even hundreds) of items of sense data a second. Of course you can't think about something and know with any certainty what the real, lived experience will be like (this is also why living out fantasies very often leads to disappointment).