Under normal circumstances, you'd be completely correct and I would expect that to occur a heck of a lot sooner than one week before an engagement.
Unfortunately, Luke apparently didn't understand what this show is about. It's not normal circumstances. Every one of the successful dozen or so relationships has had to grow without destination dates, helicopters, luxury boats and alcohol. They've had to survive and grow on real life with real life problems. This isn't normal.
Now, I'm fine with the concept of, once the engagement takes place, all outside physical contact should stop. But this is a dating 'game' where nothing is off the table. If Luke thought he could re-write 15 years of rules, he was sadly mistaken. He came off this entire season looking like a controlling bastard and he shouldn't have made it past the second or third week. Last night truly shone the spotlight on what a miserable individual he is, for all his spouting of Christianity. And the fact that, in the preview, he shows up next week declaring that she doesn't know what she wants and he's back to show her the error of her ways after she booted him out of consideration shows his complete lack of respect for her. Period.