I'm not an expert, but I wouldn't have an issue with it. I am religious and conservative in many ways, but I also do not believe in book banning, so.....maybe I'm no help at all! I don't think there is anything wrong with marbles, it is not crude in my opinion. My only word of caution is that MG can be considered for children as young as 8. I don't know if that changes your mind, or not.
I think of the Giver. I know it was read out loud in my husband's class when he was in grade school, and I read it in the fourth grade. We were probably both ten at the time, but it is an MG book, and the MC has a dream about asking a girl to take a bath. It isn't graphic or anything, but I would say it is more than just the word "marbles".
Of course, some people wouldn't let their kids read the giver. I heard a woman say she won't let her daughter read Junie B. Jones. If someone is a book banner, they'll find any reason, and it may not be the reason you think.
If it is any encouragement, I had an english teacher joke that if she wrote a book she would try to get it banned, because then people were sure to want to buy it and find out what was in it. Sounds like a recipe for success to me!