You jest, but under Australian laws at least, if the ballot is marked in any way other than to indicate your vote, it must be thrown out. So yeah, don’t get jizz on your ballot folks.
The tests of whether a ballot is formal under Australian law is primarily to make sure the intent of the voter is clear. If the scrutineer is comfortable that, for example, the numbers are legible and in boxes on the ballot paper, then it doesn’t matter what else is on it.
As far as I know there are no rules saying you can’t cum on your ballot paper as long as you also numbering the boxes correctly to indicate your preference.
I used to be an election scrutineer. The formality rules are here