A person who is high on marijuana can be taken advantage of because she may not be as aware and alert as she needs to be to recognize the danger and seriousness of her situation. Also, she may just be too weak or out of control to defend herself from assault or rape. Pregnancy is also a huge risk with unprotected or improperly protected sex. Keep in mind also that when the parents of the resulting child are both marijuana smokers, the chances of birth defects and difficulties increase. Of course, not every person who smokes marijuana is going to have sex, but it is an added risk to keep in mind.
Sex is just one factor to consider when making decisions about drugs.
Of course, there's also your health, your safety, your future.