there are many bikes out there aside from the americano that will be stiff...
basically any bike made with oversize tubing will be loads better. also, compact frame geometry will help due to the smaller (stiffer) triangles at play. My TT silkroad is very laterally stiff (columbus zona OS tubing) as is my roberts roughstuff (reynolds 851 and columbus nivacrom OS tubing). my Kuwahara and specialized expedition are both much more flexible - standard gauge 531 (or equivalent ishiwata 022). They also have traditional 'square' geometries, so even though there is less seatpost showing and smaller stem length, they flex more. one way to see this is to load the bike, grab the stem and wiggle side to side to generate wobble without letting the wheels move at all....
that said, flex isnt a bad thing (provided the bike doesnt shimmy at high speed) its more of a preference. I prefer stiffer rides for most loaded riding.