yeah, littlefinger acting is no biggie. to be honest I've never noticed anything wrong with it myself. I remember complaints here about the actor in The Wire, never understood that either. Perhaps it's a language thing (which would be weird anyway but who knows).
the rape /not rape thing, wasn't even on my mind either. it clearly was a gray situation, where the action was wrong and they both knew it but went ahead anyway. it's not that hard to portray this ambivalence (the straw dogs reference, case in point) but it's hard to digest it if you expect a clear cut depiction of either acceptance of rejection of raping in a sexual act. no one wants to hear about a sexual act which wasn't 100% something (consensual or not), but this scene depicted that ambivalence.
however, two things. one: how does this affects jamie's redemption or whatever? why would this change anything? more in point: what redemption? he got honorable with his guardian, but that's as far as that went. two: what is raping a woman in this universe? they talk about rape every two episodes, it's been portrayed as a fairly normal thing, a fact of life, something that happens. what would be the supposed consequences for this within this world? the scene seemed to be about liberating a passion both characters had been suppressing for different reasons, finally liberated in the worst setting possible. within the moral rules of this world, incest is worst than raping, and incest besides the deathbed of it's own product is the best portrayal of the character's moral decay.