Is there a reason why we use "/" for genders, and "_on_" for the body form, or is this just something that happened for no reason?

The / for genders is an outgrowth of the old "straight/gay/bisexual" controversy in which taggers would mistake those tags for characters' orientations rather than the TWYS version of the type of sex that was going on. Thus, these tags would, for example, get tagged to solo when they shouldn't and be tagged in situations when they shouldn't, like a gay male character having sex for whatever reason, with a female. Because of frequent misunderstandings, tag wars, arguments, and Takedowns ensued until those tags were aliased away to the current gender/gender tags. In this case, the / can be thought of as being short for "engaging in sexual activity with" without specifying who is doing what to whom.

The form_on_form is the result of a separate, (much) less contentious tagging project and likely is formatted the way it is to differentiate it from the more important gender/gender, assuming it crossed anyone's minds at all.