YANBU.
I adore my two boys, they are my world and I love the fact they bring me such surprises and energy. As one of three with two sisters, my DSs are such a refreshing change and I love their boyishness and the fact their clothes are permanently covered in mud (no matter what anyone says, I do believe there are differences and yes probably caused by nurture but anyway).
We are deciding whether to TTC number 3 - I'd love a larger family (again growing up one of three, I enjoyed all the benefits of having more than one sibling and the energy that comes from that). But I'm wondering, could I cope with the energy, rough and tumbles, Thomas the Tank Engine with three (and inevitable food bills of three teenage boys!!). I jest, but YANBU. It's fine to have a yearning, but you must be happy to have another boy or it isn't fair on your DC2.
My yearning for a girl also has nothing to do with childhood. Everything to do with when they're grown up, I dread being the abandoned MIL (and yes I know there are exceptions, but I'd love the relationship I have with my mum - her being at the birth of both DSs, calling her when I need her, her great relationship with my DSs). Probably also because DH doesn't have a great relationship with his own mum.
Bit of a self-indulgent post here, but YANBU! But you know you'll be ecstatic with pink or blue, the moment your newborn is passed to you, it all becomes irrelevant anyway.