So this is probably just because English is not my native. I have this scene. A woman has invited a man. They drink som wine. She asks him to spend the night together in her bed. And she says. We must not have sex. In Swedish this means what I wanted to say. It is not mandatory that we have sex. But to me in English reads that we are not going to have sex. The ‘not’ is to the sex. But in Swedish the ‘not’ would be to the must. We not must have sex would be more like it. But you can't say that. We mustn’t have sex. Does this mean something different? I can hardly have her say, “It is not mandatory that we have sex.” So how do you say this? (No worries, they will have sex in the morning.)
Funnily enough, I have this exact problem in reverse when I speak German. Any version of "Must not" implies that it is forbidden. I think you're looking for something along the lines of "we don't have to", or more compactly, "we needn't". Both imply that having sex is an option, but not a necessity.
Agreed with @NiallRoach on all points. Must not have sex means that it should not or cannot happen. It's an advisory statement. Don't do. We don't have to have sex is what you want, as Niall already pointed out. Must does not work syntactically, in any position, to achieve the meaning you want. We needn't have sex.... mm.... I'm a yank, and I don't know where your story is set or the background of your characters, but in the U.S. that syntax comes off a bit prim. It's perfectly understandable but it adds an unusual tone to the manner of speech of the character. In the U.K. that form may be perfectly workaday, but not in the U.S.
I figure needn't is the wroid you're looking for as Niall rightly suggested. There's a gap though I figure, should you ever need it, between must not and needn't (a shallow tier above shouldn't) and that's 'ought not'.
"Must not have sex" and "Mustn't have sex" means that we absolutely must avoid having sex. My first thought is, "We don't have to have sex." There are countless other ways that it could be said, depending on the character. For example, if she didn't even want to refer to sex directly, she could instead refer to sleep. ("We could share the bed. For sleep, I mean. Anything else is purely optional.")
Yeah, the phrase in Swedish carries a different implicit meaning than the same one in English, i.e. "We don't have to have sex". As others have said, I would simply write it like that. It's clear and concise, though perhaps you'd like something more colorful. You could strike a middle ground with "shouldn't", which suggests that it's desirable but is perhaps thought unwise by one or both partners.
