That's why I control what books I send home for my first grade students to read. If the parents have a problem with a book, they know they are free to call me. I have only had that happen once. A mom thought the content of a chapter book was too much for her daughter (it talked about dying a little bit) so I told her to not have her daughter read it and send it back. It was no problem. NONE of my 850+ books has any two moms or two dads things in them.
I applaud your effort, and I know for a fact it can be done...
I reviewed EVERY book on my pre-kindergarteners' book lists, and tried to make sure that my parents were informed and gave consent before we did anything "controversial" in the classroom. (Gun safety, drug prevention, etc.) I had better support from my parents than I did from my principal!