Basically, you can't be indignant about being assumed male; only about being assumed at all. This means you can't take any personal affront, because now you are criticizing someone else's style of expression, not being personally insulted or attacked.

Telling people what they can't feel when it's obvious that they're feeling it isn't likely to have the effect you want.

Sidetrack: I thought "guy" wasn't all that strongly gendered any more, but I seem to be wrong about that.