Look at her hands. Ace stole her heart.
Nami grew up with Nojiko and Bell-mere as a child. She probably feels a kind of solidary connection to other women. Call it a woman thing.
Tashigi is a kind Marine who reminds her of her adoptive mother. She remembered that time, which is why she smiled so dreamily.
The Kalifa thing is just a gag. The whole fight is pure fan service. In addition, it is not uncommon in One Piece for women to be impressed by the beauty or attractiveness of other women (Hancock, Alvida).
Nami likes money and beauty, sure. The latter probably refers more to her own appearance.
Nami is giving Vivi that look because Vivi has started to include herself as a Straw Hat and is already thinking about future adventures.
Minks behave like this towards anyone they like.
That doesn't mean that Nami might not be a lesbian after all. But I don't really see any evidence of that.