Oh biscuitlover, don't I know! I have 4 adult sons, and my sister has only girls. Over the years we have heard so much nonsense of the kind you describe.
We too saw our children as individuals, not stereotypes of any kind,and we encouraged them to blossom as they wished, and I am deeply proud of them all in their different ways.
Here are my tips:
If you do mourn for a daughter, admit it to yourself, if not to anyone else. I wasn't at all sorry to have only boys, but I have quietly 'said goodbye' to the daughter whose fate it was not to be born.
Make sure that your family has a good female dimension in it, through wider family & friends.
It is easier these days, but I made sure that our children did sports and activities that welcomed both sexes (Woodcraft Folk instead of scouts for example, but would not be necessary now)
If you love your children for who they are, then you will love those that they love. I get on very well with the 'daughters of my heart' and feel greatly privileged to have them in my life. They are wonderful young women and bring their own qualities to our cherished family.
This post demands honesty, so no false modesty - I genuinely think that they love me and value my place in their lives as well.
To people close to you, such as your mother, have a chat along these lines: if my sons are to find their way in this world, be valuable members of society, and to have their own loving families, they must know that they are cherished for who they are, so I would hate them to hear such negative things.....
To others: begin with a Paddington-style hard stare, then say "I feel very fortunate to have such healthy, happy children".
If you have the kind of sons, and daughters of your heart that I have, then you will be fortunate.