To make myself clear

W:My political science class has been teaching me about the history of women in the US.
M:Sounds interesting. Learn any new recipes?
W:No that funny actually. Did you realize up until the 1850's, women didn't have equal rights to men?
M:Sorry, I wouldn't have made that joke if I had realized it was such a recent issue.

(What are recipes here? Why it is a joke?)

The man was saying that all that women have accomplished worthy of history books is to create recipes. Then he made a remark that the 1850s were recent.

If he had realized it was a recent issue, why it became the reason he won't make that joke?

He was being a smarta*s since obviously the 1850s aren't recent.