My Dear Mistress:
I'm male and have a female coworker with whom I occasionally engage in risqué, albeit innocent, sexual banter. In the past, we've exchanged our fair share of dating horror stories - hers typically involving the feeble attempts by men to gain access to her pants; mine were more along the lines of scary first dates trying to probe my hindquarters with kitchen-like appliances. Admittedly, our mutual war stories subsided considerably after I began dating someone seriously - she's still single - though she still shares her odd male encounters, and we share a hearty laugh over them.
Until now, that is. Recently, I thought nothing of telling my coworker about a rather vivid and juicy sex dream I had - about her. Yes, I restrained myself and wasn't graphic in the least - just the broadest of strokes, as it were. The thing is, she seemed instantly offended by my candor, and now there's this weird tension between us. I have no real fear of my coworker taking legal or employment action against me - she's way too cool for that. However, I definitely want things to be cool between us again.
Yes, I am willing to apologize, but I must confess, I feel like she's being quite the hypocrite. She shares all her gross lube stories and dicey encounters with scary, nether-region piercings, but I let slip one nocturnal emission about her, and all bets are off. What's up with that shit?!
What Dreams May Come
Far be it from me, the queen of inappropriate chat around the water cooler, to scold you. However, let this be a lesson to you: Talking about sex at work is risky business.
Here's where you went wrong: Your previous cubicle confessions were always about trysts with other people. Your dream, on the other hand, was about you and her. It's inherently more intimate and brings the conversation to an entirely different level. You're admitting that you've thought about having sex with her, consciously or not. If this woman bares all about her casual sex life to coworkers, sounds like she's got some intimacy and boundary issues of her own - and now they're yours to navigate.
Perhaps she's worried you were hitting on her, despite your having a girlfriend. Or, more likely, perhaps those "broad strokes" you mentioned weren't the only ones she had in mind before you got into your current relationship. Maybe she was hoping to make your dream her reality - and now you're taken.
Regardless of her reasons, she's clearly uncomfortable, and it's time to clear the air. If you hang out with her outside the workplace, address her in person. Tell her how much her friendship means to you and take full responsibility for crossing the line (however unintentional it may have been). If you don't generally see her outside work, send her an email - you don't want to create further discomfort on the job.
P.S. You don't let first dates probe your hindquarters with kitchen appliances? You should try it sometime; you might like it.