I recently came to the understanding that I am asexual. There hadn't been any wailing or gnashing of teeth until... I realized I would have to tell my friends. They're really accepting and such, but I've already been bouncing between bi and hetero...
... for a couple of years, and I don't want to change it again, only to have them disregard me. I also don't want them to think that I'm copying the ace in our group. I already have to deal with a family who doesn't really believe in asexuality...
... and I'm scared of being laughed at or ignored. What do I do?
You don't have to come out to them if you don't want to. If you do, I'd probably do it casually, maybe in the context of an existing conversation. And if they're good friends, it shouldn't come across as copying. Groups of friends don't typically consist of "the straight one," "the ace," "the bisexual," or what have you. In my experience, people of like orientations tend to find it easier to be friends - "in real life," I have three such friends, and we all realized our asexuality at different times. So ideally, your friend the ace should be welcoming, and your friends should be very accepting.
Just be honest, make the point that you didn't change, but rather realized something you already are.