Anyone can do whatever they think is more likely to yield the results they want. Dag is saying that it's necessary because otherwise would be a "douche bag move" and "disgusting."
Why not be upfront? Because you think explaining what poly means to you in person is better than someone reacting to what they think it means, especially if they don't have experience with it. Because you can rightly assume the probability that they also are dating more than one person. My partner has personally encountered all of the above on multiple occasions. Not one person has been offended or thought they'd wasted their time. Albeit that's a relatively small sample size when compared to society in general, but a real experience is more grounded than an abstract guess any day.
The analogy of transsexalism to polyamorism, I personally believe, is flawed. One has to do with physical and mental gender issues, the other has to do with a relationship style they may in fact be emulating by dating multiple people themselves.
Again, I'm not trying to say anyone should act in any way they don't want to. You feel it's hiding something, don't do it.
Me - male, 42, poly, straight, in a serious relationship with Audrey, also casually dating.
Audrey - female, 20, poly, pansexual, also casually dating.