The only thing I would say is assume EVERYONE will find out. Think about that - and about what kind of office do you have? Very formal and corporate, or more laid-back? Friendly and congenial, or more cold and stiff? Do you feel like you can trust your co-workers or do you worry about them stabbing you in the back?
Always assume everyone will find out, and decide for yourself whether that would be o.k. Because no matter how discreet you think you are, it's never enough. Unless you really never go out in public and have a complicated system of disguises, underground tunnels, and always pay for meals etc. with cash.
Some offices, no big deal, they'll be happy for you. Other offices, not so good. My own personal experience with this was U-G-L-Y, with another bitch at the office finding out about it, threatening me with the info, figuring creative ways to tell me she thought I was a whore without really using the word directly, and then after I no longer worked there, trying to throw around gossip about me and insinuate that the office was a hotbed of sexual harassment and discrimination. It wasn't, she was just a bitch - she wasn't liked because she was a bitch, not because she was female. I was really worried for awhile that I was going to get a subpoena in the mail and be called to testify for some crazy lawsuit she was threatening to bring against the company. Bottom line: way too much trauma for someone I only dated for a few months. He was a great guy, nothing wrong with him, but just nice to hang out with, not any amazing soulmate.
As they said on Friends once "Don't dip your pen in the company ink!" (I guess that applies more to guys though, haha!
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