It's fine to be disappointed. It's not a reaction you can rationalise. It's a visceral punch to the gut which you don't actually want to feel but you can't help it. Sometimes you don't know why you react that way. Sometimes you can trace it to experience of relationships in the past.
When I was pregnant with my first and I found out it was a girl, I did wobble. Mostly because I have quite a difficult relationship with my own mother and I was terrified that I would repeat history. I didn't know how to have a close relationship with a woman/girl. I had mainly male friends and grew up close to my Dad and brother.
I can tell you a few things with the benefit of 8yrs of hindsight:
Raising girls and raising boys (I have both) is no different. They have the same need for love, care, respect and guidance. Their likes, wants and interests are guided by their individuality, not by their genitals. Really, all you actually know is that they have girl parts and not boy parts. You don't know their voice and their mind and their peculiarities and the things that make them inherently, wonderfully them.
You say you don't want to do spa things and girly stuff. Well what if you had a boy and they wanted to do that stuff. What would you do? Scream and run away? No, you nurture the individual you have. I have a little boy who loves ballet and pink and princess stuff and while it might not be my kind of thing, that's the point of parenting. You nurture them, you don't replicate yourself. Their enthusiasm for their interests is enough to sustain the both of you.
You find the sex thing melts away. You realise quite quickly once they're here that they're just a baby and the nappies and milk and sneezes and amazing hiccups are just baby things. You fall completely and utterly in love with the individual.
I am so glad I had a girl actually. I am not my mother, dd is not me. She is so much more than the sum total of my experiences. She is a blank canvas, ready to be her own person. She taught me that the wary, inherent mistrust I'd manage to develop where women were concerned was bloody ridiculous. MN and its brilliant women and this tiny girl I produced showed me that I was wrong. It was the best way to learn that I could have a strong, positive, wonderful relationship with another female. She taught me that.
You'll be fine. It's okay to feel a little grief for the boy you imagined. Then celebrate the girl you will have. She will change so many of your misconceptions.
Don't let people tell you your disappointment is wrong. I know you don't want to feel like this. People will say at least you can have a baby or at least your baby is healthy and yes they're right but of course you are mindful of that. This is a separate and unasked for worry.
It will pass. I promise. You're in for a great time. A third baby is such a gift and she'll be the strong, loving relationship with another female which you're missing out on.