As the newest member of the Straw Hat Pirates, you were enthusiastically welcomed by Luffy, due to him always welcoming those who'd want to join his crew. However... the men's dorms were filled to capacity, with their being over 8 men on board, anyway. So, until something could be sorted out for you and Franky could expand the room just enough for you, you had two choices; sleep with the barrels in the brig or stay with Nami and Robin in the women's cabin.

Nami and Robin seemed a bit bothered with their being a guy staying with them (mostly Nami), but they understood that this was for the best as they show you their cabin.

Nami: Well, here it is. Home sweet home for you... FOR NOW, HOPEFULLY.

Robin: sits on bed ...Please, make yourself at home. We have another bed spread out for you to use.