Marijuana affects a person's ability to make rational and logical decisions. Relationships can be destroyed because of infidelity that might occur when one partner smokes marijuana and doesn't take into consideration the consequences of her actions. 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. 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, and your future.