gentlemen, think like a getbigger.
your man seed is worth a small fortune. think about 18 years of child support. think about what kind of mom she would be to your child. And, think about what happens if she's shady/crazy... she goes home with panties full of jizz, her man discovers it, and suddenly "I think this guy slipped me a mickey or something, took advantage of me..." Suddenly you have unknown dude wanting to murder you. Or she cries rape after begging for it, and that DNA is aplenty.
Have your fun, and bust nut into your strategically placed boxers. Drive home commando with piece of mind.