#Magical Sexually Transmitted Diseases! 

I like the answers so far but just as a alternative view, you could introduce STDs that are magical in nature. They could affect stats (Cha, Con, Str), they could affect AC, chance to hit, or sometimes interfere with their actions, cause madness, they get tongue-twisted while casting a spell, or space out and drop their weapon. :) 

...if they have babies, get them to pay an in-game version of tax on their gold. 

Have fun with it.