Sperm cannot penetrate the fabric of your clothing. Any semen you ejaculated stayed in your shorts.
The vagina is located inside a woman's body. You were rubbing against her genital area, which was covered.
I strongly suggest you and your girlfriend read these articles:
"Answers for Women: Am I Pregnant After Dry Sex, Making Out with Clothes On?"
SHARE:"Outercourse: Dry Humping, Dry Sex and Other Sexual Behaviors"