I think getting hung up on a label does no one any good. If Liz is supposed to be a LI in this game, then she is what is considered to be 'in play'. This means that depictions of her in the past that would constitute "vague term here" if encountered in the present while she is an active LI of the MC will still render a similar response, sentiment, etc. regardless of what you intended,
DV FiSTer.
I see no reason why in the case of Liz it was that particular scene with that particular setup that was needed for the flashback. Why not the perving on your mom be when she is fucking your dad? That is much more likely to be something that could have happened, rather than the MC somehow being in the same house with Liz and some random "Very stereotypical BBC" that canonically is almost exclusively used in titles with the NTR tag.
If you are trying to pull something and be coy with it, just remember that you are barking up the wrong tree here. People that want that content want to be able to search for tags that have it, just like people that don't want that content want those tags to be there so they can exclude it in their search.
If you aren't trying to be coy then take the feedback as it is intended and don't worry about the particular vernacular used to try to convey said feedback.