Farout, I'm just confusing myself now.

If I was right before, and I consider all three possible outcomes (BB, BG, GB) equally likely like I did right at the beginning, this suggests that there's only a 1:3 chance of any particular child being a girl, which doesn't sit right with me.

Alternatively, if we say that the likelihood of any one child being a boy or girl is equal unless the conditions of the challenge prohibit it, we get this new tree:

Which suggests only a 1:4 chance of getting two boys from the woman. Of course, this doesn't sit right with me either.

So now I try to unthink everything and go back to the beginning, ignoring the man's situation:

The woman has a boy. She has one other child.

What are the chances that she has two boys?

This is the same as,
What are the chances that her other child is a boy?

That is an obvious 1:2 == 50%.

By considering the man's situation I was fooled into considering order for the woman's situation, which is actually irrelevant if I read the question ignoring the man.

So I think you are right

Jibb

EDIT:
@Hummel: Boy do I wish I had the motivation. I just removed Sculptris from my computer since not only was it crashing regularly, but a whole bunch of other problems with my otherwise-flawless computer cropped up around the same time I downloaded it, so I'm feeling particularly loathsome towards it. At the same time, I miss having a sculpting program with a "crease" tool.