There are 2 things going on here. One is about what is fair and equal and most likely to lead to social mobility for everyone and the second is about what we want and will do for our own kids (which will probably give them advantages over others). Although we might like to think that these two things are one and the same they are often in conflict.
So when I say I want my kids to be challenged and not disrupted by others I am saying that I do not want my kids to be educated in the same classroom as kids who are likely to be disruptive and mean that my kids can't work at the pace of which they're capable (unless I have guarantees that the disruptive kids are going to be prevented from disrupting and the kids that need support are given it which I might have if class sizes were reduced and there was more specialist teaching, training in SN and behehaviour management and in and out of class support) . So as a middle-class parent who is fully aware of the strenghts and weaknesses of my local schools (dp and I have taught in lots of them) and able to read league tables and able to work the system one way or another I have choices
I can:
move house to be in the catchment of a school were disruption etc is limited
adopt a faith
go private
send my kids to the local comp anyway and support their education myself
etc.
But I recognize that those choices are not available to everyone. Even by having these choices my kids have an advantage which not all kids have. And taking any one of those choices adds to my kids' advantages.
If every middle-class parent makes one of those choices then that leaves sink schools (where the kids are doubly disadvantaged becuase they're all lumped together with limited aspiration and self-esteem and teachers don't want to work there etc)
So there you go. Most people want a good education for their kids and what constitutes good is likely to depend on your own educational background and aspirations and hopes that your kids will match or surpass your own success (academic and otherwise) and happiness or surpass it.
So do I think grammar schools are right? No, no, no. They add to the advantages of middle class kids and compound the disadvantages of the poor kids and they use everyone's taxes to do this.
Would I send my kids to one if there was one in my area?
Probably.
Does that make me a hypocrite? Probably but at least I'm not saying that all the kids who don't get to go to the grammar shool are or should be happy with their lot.
You are not wrong IMO for wanting the best thing for your own kids. This is a basic parental instinct. But you are wrong for pretending that what is best for your kids makes life better for everyone because it often conflicts with it.