Ah I was hoping there'd be a topic on this, it was something that was turning the wheels in my head last night, among other things.
Anyway...so they did take this "no rape" policy of Negan's from the comics and have kept true to it here. I think there were a few different things going on, as far as the scene specifically -
I agree w/weallgotit, that show of force by Negan was first and foremost a display of his power, and a tactic to try to convince Sasha to join him, show her he wasn't such a "bad guy". I wouldn't have put it past him to have set that little scene up with other Saviors, without David knowing, so Negan could swoop in and save the day. I have a feeling he'd have done that to any man in his group who tried to rape a woman, but, without all the exaggeration.
Still, in maybe the SLIGHTEST of possibilities, I think he wanted Sasha to know that he does not tolerate that behavior from his men and that she wouldn't have to worry about it happening to her. But I also think he used that to try to get her to let her guard down and make his way look more appealing.
Now, on Negan and rape...I'd like to address the situation with his wives because here is where I think his whole stance on rape is majorly skewed. To expound on your point Weallgotit - he makes it so that the women he chooses as his wives either do that, or they have to do some horrible job around the Sanctuary that's so deplorable, or some other threat, that being his wife looks to be the only option to stay alive and live comfortably.
We saw what happened with Sherry and Dwight, so I also think he does this to keep some of the men in check, that if they go outside his rules, either they or the man they're connected to pays the price. Also saw this with that Matt guy.
To me, in regards to rape, making it so the woman has no choice, even if he's not physically forcing her down, is the same thing. She is not really saying "yes, I want to have sex with you", she's saying, "I am only doing this so I (or my loved ones) don't die." which to me, is "no" because we know they wouldn't willingly want to have sex with him being the kind of person he is.
I think Negan tries to justify it to himself because he's NOT physically forcing them like David was going to Sasha, he likes to pretend they have a choice even though they really don't. He figures they'd really choose the other way if they didn't want to be with him.