>didn't play the game
The character chosen at the start is Lucas or Dawn, the sidekick found in the past is either Akari or Rei, depending on the player gender. The implication is the sidekick in Legends is the ancestor if the sidekick in DPPt, who isn't related to the MC in any fashion, thus it's perfectly fine to fuck.
The ethical problem is does this in fact change the future where you ARE related to her, by manner of fathering her Father/Mother or if events transpire as they would have anyway (implying a stable time loop, as nothing got changed from then and now) and you're still unrelated to the Professor's assistant, thus have free reign to fuck silly, like you had in your past from Legends.
Why does the simplest of time jumps always fry simpleton minds?