A girl can give you STDs including AIDS, as well as unwanted grandkids, pot can't! I'm likely older than your parents, but not as conservative! Are you using protection (a condom)? Doesn't matter if she's on the pill, use a condom! Getting stuck with child support for the next 18 or so years is NOT how you want your life to go!

As for the pot use, print this up! Current and former marijuana use: preliminary findings of a longitudinal study of effects on IQ in young adults -- Fried et al. 166 (7): 887 -- Canadian Medical Association Journal
Basically, it says that MODERATE pot use makes you smarter! (Not so for heavy pot use). So convince your parents that you use it only occasionally, and you're home free on this point!