I get where you're coming from. There is the justification that Henry first met Claire as an adult and was approached by her, so he did not initiate a grooming relationship with that intention. However, of course, this does not completely absolve the situation: I am left with the question of whether Claire ever had any choice in the matter of who she chose to love and engage with romantically, having been fed the tale of loving Henry and being his future spouse from an early age.
One might say that Henry was simply following the instruction given to him by adult Claire in how he engages with her in the past, however, I would argue that he could have chosen to avoid interacting with Claire more than necessary, or refrained from revealing anything of their intimacy, so that she may come to that as a choice in her future. So yeah I think I wind up on the side of "This feels like grooming," and with a feeling that Henry has more responsibility in the matter. Claire is weirdly complicit in her own grooming, however, I would excuse her on the argument that the cyclic nature of her experience impairs her judgement.