It's not non-consensual if Kathryn wants it. In that particular scene Kathryn believes that only her and Luke are in the glory hole area, so naturally she believes it's him, and in any case she's currently following a non-cheating path, so I think she'd be pretty upset by such a scenario. But in the future, when she finds out about Luke's betrayal and a netorare route begins, she won't feel the same way, so a similar situation is likely to result in a very different reaction from her. Yes, it has to be that way at the moment, but all he has to do later in the game is have Kathryn realise that it's not actually Luke, and have her go along with it, and it's no longer an issue.