Well, exactly. Seven and Ace's relationship is so unique and resonant (and with that dysfunctional kink that seems to run through Seven like the writing in a stick of rock) that physically shipping them seems to cheapen it severely (as well as being kind of creepy, what with the teacher/pupil, father/daughter vibes going on; not that there wasn't a subtle creepiness to it at times even in the TV episodes, as vonquixote points out).