I actually request "no families", and even before this was an option, I was clear about it. There is a No Families request option in your preferences.
My listing, however, is in NYC, which is an often-family unfriendly city. In any event, my building is very definitely family unfriendly - and there are noise issues. My neighbors could be disturbed by family noises, and it is hard, if not impossible, to suppress the noises of each kids visiting the big city.
In effect, this is a noise, and a space issue - it really isn't large enough for families. I turn them down, with thanks in their interest, on every single occasion.
Some hosts are simply more comfortable with adults and singles - I am one of them. I also turn down anyone who raises any sort of red flag for groups, extra guests, and partying - one of the reasons I would never select the Instant Book option.
It is entirely up to you and your good judgment as to who you accept. If I had a big vacation lakefront house in Vermont or Wisconsin, I would say SURE to all families. I don't, though. You know what is best for you, and if you are uncomfortable, rest assured it's best.