What about your reputation? I couldn't imagine risking my reputation and the professional image that I try to uphold with a moment of weakness. If you have a "special" partner then I am pretty sure you both have time off, take of those needs then.
Over the years I've heard many a story of getting kinky on the job. I've seen a 50 year old man get caught hooking up with a 18 year old medic, and a couple being caught in the back making the 'ole rig shake.
I think relationships happen in every job, but some jobs provide privacy and alone time where more is bound to happen. I don't think it's right or professional, but our industry has many people with varying levels of ethics and morals.
Two entire shifts were fired for this at small residential fire dept stations here. As a variation, some more were disciplined for picking up fire floozies and hookers on/in their engines.