DD2 and DS are four and sixteen months respectively. I have a friend who has a DD and a DS of around the same age and hers share a bedroom.
I once said (perhaps a little bit carelessly) that if DS had turned out to be a DD3 I might have had them share and she seemed to get a little defensive (her house is a two bedroomed one.) I only said it in quite a casual off the hook comment by the way - it wasn't said with any judgement.
Since then she has bombarded me with information about the benefits of siblings sharing. My view is that if there is no choice obviously they must share but since I do have a choice I've chosen to keep a boy and girl apart - do the rest of you think I am mad, as she appears to!