Just ensure that all the glyphs in the font have the same width and then FontForge will automatically mark it as monospaced for you. It is not sufficient to mark the font as monospaced (Element->Font Info->OS/2->Panose>Proportion field).

If you are unsure whether all the glyphs in your font have the same advance width use:

1. select all glyphs (e.g., with ctrl-a)
2. click "Element -> Find Problems... -> BB"
3. "Check Advance" = 2048 (Make sure that the correct width is set)
4. all glyphs with an incorrect width are going to be pulled up.

As soon as all my glyphs had the same width, Fontforge automatically changed the Proportion field to be "Monospaced". "OS/2 -> Panose -> Proportion"