However, it bothers me that a type of consensual relationship is disparaged as wrong in general, when it's something that the people involved agree to and find workable.

Agreed. While the OPP is a common failure mode, it doesn't apply to all relationships one-man/multiple-women relationships. Just usually.

Also, does the one-penis policy objection apply to polyfidelity where there is only one male in the relationship? That situation seems more equal.

The objection really only applies when it is a deliberate policy, as opposed to situations where the genders of a relationship's participants just happen to fall out that way. IMHO, of course.