Into the biofilters until some poor lower deck person empties it:

Literally the first episode of Lower Decks has someone off-hand mention having to empty the holodeck cum trap.

Here.

In universe the holodeck generates a certain kind of matter that can’t leave the holodeck. So your jizz goes into the holomatter lady who is then presumably disinterested atom by atom just like a teleporter. So your jizz gets dissolved into the ether.

Into the hussy