My then-14yo, now 15yo, reacted badly when we told him. He's still somewhat negative, though grudgingly accepting most of the time. He has said, "Love should be between two people" and indicated that he's embarrassed about our relationship. It's my fault, really--we had polyamorous friends and I always felt that while that might work for them, it wasn't for me. Now I'm eating my words.
I can't think of how I would have handled it differently. I think you did a good thing with that link, and I think I'm going to have to send it to my son.