I'm not sure what you were expecting, DAI ESRB description is not that much different from DA2.
Also, we don't know what happens with Cass or Josie, both of these ladies haven't been hinted to be the vulgar characters and going by Cassandra's face below...I really doubt that she is.
*snip*
Actually, the ratings are quite different between DAI and DA2. We no longer have gore, but we have: nudity rather than partial nudity, strong language instead of language, and intense violence rather than just violence.
That's true. It is possible the female in question is the Inquisitor or even just an awkward walk in situation like both previous games had.
The scene got cut before she ever goes down on them but it's clear implication. 
If you want to imagine it differently, that's fine.
I'm not saying it isn't possible, just that there is no evidence she does. So like you said, it is up to the imagination of the player.
Personally I think it doesn't make sense for Liara to considering she's never thought of that stuff before meeting Shepard.