This was a pretty un-sensitive response to a serious topic. You may have intended for it to be a light comment, but it wasn't taken that way by the person with the issue, and I would venture to guess that anyone else who read it wouldn't think it was appropriate either.
I think you should consider a heart-felt apology.
On the subject... I had seen on Oprah of all places that there are actually weights that are made for strengthening these muscles. I did a quick search, and found this: