I'm a 35-year old gay male and I've fallen hard for a co-worker (let's call him Joe). I've known him a little more than a year and over that time my feelings just keep getting stronger. So far, he doesn't know how I feel. (Well, I guess maybe he could, because I often feel like I do a lousy job of hiding it, but if he suspects anything, he hasn't mentioned it.)
The problem (other than me being shy and terrified) is that he is involved with someone else. Exactly how involved is difficult to determine. He lists himself as "single" on an online profile, but he definitely has something going on with this guy ("Ted"

. A few nights ago we were at dinner with some friends and he got a phone call from Ted and said "I love you" before he hung up. I felt really sick. So anyway, based on the limited evidence, we can say that they are more than casually dating, less than co-habiting.
So my question is this: assuming that I could ever find the courage to tell Joe how I feel, is that ethical? I've asked a few friends and got widely differing opinions, ranging from "all's fair in love and war" to "keep away from anyone who's seeing anyone else, period" My case with Joe certainly won't be helped if he thinks I'm a sleeze bag for even approaching him while he's with someone else. Needless to say, "Ted" isn't my favorite person right now, but I don't want to risk hurting him either.
So do I have a talk with Joe (and if so, how), or is the only moral course of action to suck it up and keep quiet?
Any advice greatly appreciated.