K9 semen is mostly water, studies (which I can provide if you're interested) show that by volume its 93-97% water. The body will absorb water quite easily. The vagina is a mucus membrane and its able to absorb water very easily. The remaining contents of the semen (Proteins, and other nutrients will either be absorbed along with the water or will just remain in the vagina until it's eventually excreted as the vagina does its natural process of cleaning itself.
A moon cup is designed to prevent leakage of fluid and tissue during a womans menstrual cycle, as a result it does a pretty good job of keeping his sum inside me. Also since we primarily only have sex in the evenings, I'm only around the house anyway. I have never noticed any running down my thighs once I insert the cup. Initially there's always some leakage when he first pulls himself out, but I try to keep as much in me as I can. Whatever does leak that he doesnt lick up I wipe off with a towel.