A jizz monkey is in fact a job description and refers to the poor unfortunate employed to clean up the inside of peep show/ sex show boothes, so as said employee wipes spunk from the walls he is a jizz monkey. I suppose it's like the equivalent of being a runner for television or something. Admittedly it is not a particularly desireable line of work, but I hear it pays well...
Bill: 'Has Bob got himself a job yet?'
Ben: 'Why yes Bill, he's working as a jizz monkey down in the red light district'
Bill: What? He gets paid to clean up spunk, that's just fucked up man.'
Ben: It's a dirty job, but some ones got to do it...'