Ooh I've been there.
Boys terrified me. I've been a girl, I already had a girl. Boys were that complete unknown (Psycho thinks her boys scared me, but it wasn't that, it was just that completely different species thing).
When DS was born it did take a while to adjust - I had always had this image in my head of two daughters (all my side of the family have 2 girls at least for the first 2), and there was an element of grieving for that lost child.
Don't get me wrong, I loved DS, but he wasn't what I had pictured.
I then had one night, I saw him sleeping in his crib, and I suddenly welled up with love for my son. Not my baby. Not the non-girl. For my little man.
He is so so different from DD - he bashes and he climbs and he turns everything into a sword (even at 22 months), but he is loving and caring and has the most mischievous grin that he gets away with way more than DD ever would have.
I remember talking to my mum about some mythical future with my DD having a relationship like I have with my mum (yes, I know I can't predict that, but it would be nice), and how you don't have that same girly closeness, and she pointed out the benefits of a big manly son to come and give hugs etc.
If I go for a third, I think I can honestly say that I would be happy with either sex - they have their pros and cons. When DS is climbing up bookshelves I think a girl would be better, but when DD throws a hissy fit because her hairclips are wrong I opt for a boy!
You feel guilty for feeling like this. You will feel guilty if you have a boy during those first few weeks getting your head round it, but you will get your head round it. I wouldn't swap my little man for anything in the world.
(well, I lie - right now he is on offer free to a good home so I can get a few hours sleep )