Refers to the extremities of sexual deviations (paraphilias), e.g. shit fetish, snuff fetish, fisting, etc. The fact is, the human mind can be distorted so much that there really is no limit to sexual deviation. So the limit is in a metaphorical sense.
You may think the sexually disturbed sickos imprisoned for life in max security prisons have reached the limit of sexual deviation, but how can you be sure?