I think the Canadian approach, if you like the idea of stepparent liability for child support, is not as bad as it could be because it appears to treat the stepparent as more of a surety, a secondarily liable obligor, with the natural parents primarily responsible. Somewhat complicated but not an unreasonable approach.

Again, I would question the public policy behind making stepparents liable to any extent for the support of their spouse's children. I doubt very much that most people expect this liability to be imposed merely because they have married someone with children and many, if they were aware of this, might not wish to marry at all as a result.