Exactly what I was thinking. Given the pep talk from Baelish about "making Ramsey yours" I would have thought this scene would have played out very differently. Add in Sansa's talk to Myranda and there would have been more than enough precedent for her to seduce/control Ramsey in this moment.
Also, as Sansa, you know this is coming. You know the customs, you know Ramsey is a sick person. Mentally, tactically, logically, (post Baelish) Sansa would prepare herself. The show has not been leading Sansa to be this dumb and it sucks to just have everything thrown in the gutter.