Long as you wipe it off before it would slide down your stomach to your vagina your good. Just don't let it get in there and your fine, also cum can only last so long outside the guys and girls body then it dies. Less it is frozen, or put in a fluid that is the right temperature to maintain it, and even then it will only last so long without nutrients. SO, it's not like it will last forever. If I remember correctly it doesn't last long if he just shoots it on your stomach. Not long at all. I'd look it up, or maybe someone in here knows but I remember it's not very long at all. Which is why the guy normally cums in a girl to get her pregnant and your vagina is a certain temp all the time, it's a homeostatic for growth. Hence why bacteria love your vagina as well.
Hope I helped my advise. Just wipe it up with a towel. Or, use your hands and then eat it up lol. By the time you get to the shower it shouldn't matter they should be dead, like I said they don't last long I know.