There's violence and there's hraka (if you know what that means in Lapine), so this book may not be totally suitable for young children. (Although young children can be pretty violent and hraka-oriented.) But there's not a lot of sex in the book. In fact, there's no sex or even rabbit-smooches here. So we give it a PG.
So why isn't this G, like The Lion King? There isn't a lot of sex in the book, but a significant part of the plot is about finding enough female rabbits to mate with; and how they need to mate so that the warren can have a future. It's fair to say that these rabbits have mating on the brain. (Just like real rabbits.)