Are there also pictures of gay people having sex?
You picked one book that apparently you think is fine for K-3rd grade. There are also books that show images of naked boys and naked girls. They have images of sexual positions.
Your book may or may not be okay for small children, Im guessing you (or the author of an article you read) picked one of the safer books for your example.
The much, much, much bigger point is that sexuality of any kind does not belong in public schools. That is just not the place for any of that. Just like movies like Texas Chainsaw Massacre or Night of the Living Dead does not belong in public schools.
Let the parents of these small children keep their right to protect their children for seeing objectionable material of any kind. It doesn't matter how sanctimonious or self-righteous the leftist is, our people's children are not yours to indoctrinate.
That means if a parent wants to keep information about gays and lesbians that does not contain objectionable material then it is not your right, responsibility or obligation to expose that information to their children. It makes not difference how beneficial you think it is, you just should not even try. They are not your children. They do not belong to the state, they belong to their parents or legal guardians..
School must be limited to just the basics of education. Reading, math, writing and science. Nothing else until the children are much older. Public schools are NOT leftist indoctrination camps.