The kid is 3 but nearly 4.

And the mother claims she will breastfeed him until he reaches 5.

I don't really see how it is "sexual" at that age I can see why people might think its inappropriate though.

I personally think its her choice. The bigger problem is that she is plastering her childs face on the front of a huge magazine, literally on her breast, for publicity and as a tool for controversy. That isn't going to be nice for the kid growing up.

He too old. Gross!

There's nothing more natural than that. Granted, the kid looks a little too old to be breast fed and raises questions about over-parenting and attachment issues, but breast feeding in itself is perfectly natural.