Why in the paradox of a boy and a girl, the outcomes of DM and DM are not the same?

Question

Mathematics

The boy-girl paradox analysis talks about four possible outcomes before the first test: MD, MM, MM, DD
Why are MD and MM different outcomes?
In my opinion, the initial outcomes are: same-sex children (M), same-sex children (D) and opposite-sex children.
Where am I wrong?

Answer 1

Because some child is the first, some is the second.
The birth of a child is an unrelated event.
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