Not seeing how it can help them is justification for nanny state bans like this? Does sitting on Santa's knee help them? Why not ban that?
I'll tell you how it can help them. It can reinforce the notion that being gay is not an abnormal part of society which should be shunned and kept in closets, important for the mental development of children.
While I hate anecdotes, here is one which might illustrate just how harmless this is. I very clearly remember that when in kindergarten my teachers had a cow because I wanted to push the prams around a concrete path with the girls instead of plating football with the boys. Quite simply, I liked the girls. I grew up straight as a die so the experience was obviously not formative or negative in any way.,