What does "なんだもんなあ" mean?


> 同じ部屋とかもう慣れてるはずなのに"

> 当たり前のようにダブルべっどなんだもんなぁ

In the first line the character is like "We should be already used to sleep in
the same room and yet"

By context I can gather the character is embarrassed to sleep in a double bed
with their boyfriend. However, I'm stumbled by the actual nuance?

"Like it's natural, we got a double bed?"

