It is normal for you to have a discharge. The whole inside of your vagina will lubricate when you get that aroused. That will make your panties wet. Some women will get soaking wet, some damp. You must have noticed it before?
Sperm are just not that good a traveller. If it is on the outside of your panties, it will have to get through it, find your vulva, then find your vaginal entrance and then swim all the way up to your cervix and into your uterus and Fallopian tubes. And there must be millions and millions of sperm making that trip successfully and staying alive and healthy. Then you must ovulate at exactly the right time and one sperm needs to be strong and healthy enough to fertilize your egg.
Such a thing is just not likely at all. Please be careful. He should never press his penis into your vulva, even if you have your panties on. You can rub on his penis all you want, or you can use your hands, fingers and mouthes, but if he does not have a condom on, keep that penis head out between your lips. Only stay outside.