As long as the semen does not soak through the layers of clothing and onto/into her vagina, she cannot get pregnant.

The chances are very small but why take the risk? You might get so excited that actual intercourse is just a small step away or the panty can slip away without you even realizing it.

Put on a condom or keep the penis away from the vulva.

Fingering, oral sex and hand jobs are just as much fun (if not more fun) as rubbing a penis against the girl's panties.