Chichi was not 15 and neither was Goku, they were 19 when they got married and soon after they conceived Gohan. Remember Gohan is 4 years old when he is introduced.. which takes place 5 years after Piccolo Jr. is defeated at the Budokai Tenkaichi.
Well, even if it did, it would be okay. In japanese law, I believe I read that age 14 is the age where it becomes legal to "Do the deed." Not to mention that they were married. If the father gives their children away, anyone can marry at about anytime. Now, I'm far from saying that I agree with this at all, but it's the law. So, weird or not, it's perfectly okay in Japanese law.
At least Kid Buu LOOKED like a Kid in comparison to the other Majin appearances. Gohan being called “teen” in Cell saga extended to him being called “adult” Gohan in Buu saga as a posed to teen, y’know since he was an actual teen then.