Assuming you’re disease free (and so is your partner), not much, really. You won’t get anyone pregnant and, both being disease free, you won’t be sharing any nasty bugs between yourselves. So all you’re doing is wetting the insides.
Edited because the original question answered is different from the current one: “When sperm enters the anus, where does it go after that?”
A bit of the water is absorbed by the lining of the rectum. The remainder will eventually be moved out with the next regular bowel evacuation. The conditions in the rectum aren’t what the sperm need to want to travel upstream to a non-existent egg.
Actually, the most quantity will come out in liquid form sooner than latter. But a small quantity will actually be absorbed by blood vessel into the blood stream.
It´s called Rectal administration - Wikipedia
But think of it like how much sperms comes out, say a spoon full out of which 10milligram would be absorbed