I remember feeling a lot like you when I was younger. I have a large PWS on my entire left upper leg and part of my calf. I used to hate wearing shorts and almost never did in high school. I'm now 25 and have (mainly) come to terms with my birthmark.
I can tell you that even though I was embarrassed by it, my birthmark didn't affect my dating life. I would explain it to people, just saying it was a birthmark and I had it since I was born. Not one person ever cared that I had it. I even asked my current boyfriend of 4 years about it the other night, and he said he noticed it when he first met me, thought, oh, she has a birthmark, and that was the last he ever thought about it.
Anyone that is a good enough person to be with you won't have an issue with it at all!