Simply say, "I like just having fun and talking to ALL the girls at work. It makes the day go faster and it makes the place happier. I'm not looking for any kind of relationship here at all. In fact, I'm seriously involved with someone and I'm considering marrying them." By saying that you like and enjoy everyone doesn't exclude any particular individual. You will take tremendous pressure off of them by showing them that you were never attainable to begin with. Try to come off as the guy that likes and respects EVERYBODY. This will keep things healthy. It's when lonely, unstable individuals get rejected forthright that things get a little on the sick side because they NEVER felt like they belonged to humankind to begin with. You will only confirm this feeling by pushing them away. The trick is to embrace ALL - theoretically. If you flirt with one, then you must flirt with all - this will get you off the hook with that solitary person. Remember, "Oh he's alright...he likes all the girls." You're better off being very generic instead of specialized. This attitude DOES make those people who occupy the fringe of the social circle to feel as though they belong. When it comes to the work environment, a lot of people who start a romance or crush on a co-worker do it for status. It sets them apart from everyone else in the office temporarily. They use it to shine because they may not be able to shine through their accomplishments on the job. It may be a mundane situation for them. When I was a teen, An older, married man I worked with would compliment me a lot. I would say the old phrase, "Ohhh, I bet you say that to ALL the girls." He'd say, "yes, that's true but only to the prettiest ones." So, he established his boundary and gave me a compliment just the same. There was harmony there. No infidelity, no flings. Just acceptance. It was nice. 
Last edited by RightGalaxy on 28 Apr 2014, 8:37 am, edited 4 times in total.