In addition to the above I have managed to do defile no fewer than three offices, but not on work time.
First time, while I was having the aforementioned colleague-with-benefits fling, her best friend in the office, who didn't know about said fling, came onto me in a big way. She was lovely - Iranian extraction, looked like Princess Jasmine. While working late ended up doing the naughty on her boss' desk chosen by her has she haaaaated her boss. Ended up dating her about 12 months. She even bought a Princess Jasmine costume. Probably should've marri3led thst one.
Second time was a Spanish colleague. We had a habit of going to networking events, having a few, stopping in a pub on the way home for a few more and some intense snogging. She reckoned she had split with her boyfriend, but never wanted to invite me back so I was dubious. Anyways, one night after a work do we both had to go back to the office to pick up our laptops at about 10pm. We were fooling around being stupid and security came to see what the commotion was. The moment they left she was on me and ended up doing the naughty on my desk. Next day she says "I hope I'm not pregnant. I'm not taking birth control" which was pretty terrifying. Glad I didn't marry that one as she was craaazee.
Final one, I had been dating my Mrs for about 3 months. Between Christmas and New Year we were pretty much the only ones in the office so we got into a tradition of doing the naughty at lunch time. And I did marry that one.