The ONLY way that this might make any sense is if it's defined as: attraction to a single gender... but both sexes.

So, for example, someone who's attracted exclusively to men and women with "feminine" presentation/behavior (in gender-role terms).

The problem, of course, is: 1.] that's almost certainly not the term's intended meaning; and 2.] there's really nothing "semi" about such a person's bisexuality! They're just a particular flavor of bi-- bisexual with a gender-related preference-- rather than something else altogether.