I don't mind you asking about it.
The first time I was approved, social services did actually prefer that I adopt a girl - when I expressed to my social worker that I felt more comfortable with a girl, she was pleased and so was the panel etc. However, I was approved in 1995, as a single lesbian mother, and so you should view SS attitude in that light. Things are different nearly 20 years later, but in the mid-90's, there were very very few adopters who were gay, and few single adopters either, and the professionals tended to think that a gay women should raise girls. They would have let me be approved for a boy had I insisted on either or boy only, they wouldn't have stopped me, but they were more comfortable with me adopting a girl. In 2014, I would not expect any reasonable professional to hold those attitudes.
As it was, SS opinion wasn't an issue, because I also felt more comfortable with a girl. I felt I could relate more, was more comfortable around girls, didn't feel very confident with 'boy issues'
Looking back, I would have been fine with a boy. I was not thinking realistically, I was not confident in my own parenting abilities. I would have been fine with a son. But of course, if I had my time again I would adopt my DD1 all over again!!
The second time I was approved (7 years after adopting DD1), my decision to only be approved for a girl was based solely on my existing family dynamic. I was going for an older child again, anywhere from 4 up to an 11 year old, and my DD1 did not feel very comfortable with the idea of living with a brother who was that old in the house at that point in time. Also, I now had 6 years of experience with daughters, 6 years worth of toys, some clothes etc that could be handed down (I'm not well off by any stretch of the imagination). An older girl just worked much better for us
By the time DS came along things were different as DD1 had moved out etc, and also a sibling of your children is very different to being approved for an unrelated child