Adapted from a recent online discussion.
Question:I liked your answer (http://wapo.st/Rtbmfx) about the boyfriend struggling with his girlfriend's sexual past. One thought: I think it is cruel to say, "I used to do that, but I won't do it with you." I once had a girlfriend tell me she enjoyed oral sex, but wouldn't do it for me because she thought of me as a potential husband and "wives and mothers just don't do that." That was just mean. If you don't want to do something, just say no, don't say you used to do it but won't anymore.
Answer: It's not automatically cruel to indicate you've closed a door on a past behavior. Someone could say, "I know from a past relationship that I'm uncomfortable with X." Though I would still advise saying even less: "I'm not comfortable with X."
But that's incidental to your story, which isn't about being cruel; she actually did you a favor. She revealed that her worldview (a) differed from yours in an important way, and (b) is seriously warped. What a misogynist sexual line she drew, slut-shaming women who take pleasure in sex.
The best time to find that out about someone is as soon as possible. The best way to find that out is through frank conversation. Forget doing you a favor; she gave you a gift.
Comment: I can think of plenty of things I tried that I wouldn't do now - and it's for precisely the reasons Hax suggests. If someone told me they'd done it but weren't comfortable doing it anymore, I'd respect that. And if someone told me I was being cruel by not giving up all the same sexual favors ... well, I don't owe my current partner all the sexual experiences of my past; that smacks of something really ugly to me.
Answer: Yep. Expecting is ugly. Judging is ugly. Accept each other as is or move on.
Comment: It's really, really easy to tell someone about things you used to do even if you won't do them now. People ask. It's really easy to go from "How many people have you been with?" to "Have you ever done X?" And what are you supposed to do, lie? If anything is cruel, it's asking your partner about their sexual past and then holding it against them when they're honest.
Answer: A wholehearted yes to the last sentence, though what precedes it sounds disingenuous, or at least shortsighted. You can say, always, "We're adults. Our experiences brought us to each other. I'm comfortable skipping the show-and-tell and hope you are too."
And if s/he's not, then you need to ask yourself why someone insists on digging; I warn strongly against taking any interrogation as a given.