I also do think that it needs to be taught that sex should be enjoyable and never uncomfortable or dissatisfying.
I do feel this is the key point for any teaching, and I would add to it, never painful or if you don't want to for any reason.
I think then that many contentious parts of sex education can be taught as things that some people do and information on what it is, such as oral sex, anal sex etc. but that if they are done violating any of the above, no-one has to do it.
I think something on the lines of 'only if it is enjoyable, not comfortable or painful, what you really want to do' should be repeated over and over again throughout the teaching. I am thinking of children who maybe being abused, and hiding it away because they've been told 'it's fine' or 'everybody does it' by their abuser. To hear this 'only if ...' might enable the child to tell someone. No need for lessons on how to do these activities, just what they are and what they are called.
I do think masturbation should be talked about in primary school. All those poor children in the past who were taught that it was wrong, if you were masturbating you were doomed to going to hell, to lose that part of your anatomy, that you should be ashamed!
These teachings are (I hope!) are well in the past, but I think it would be helpful for children to hear that it is a normal part of life and not shameful. Again no need for instructions on how to do it!
I can see a similarity to how schools nowadays teach religion. Even in the C of E primary school I worked in, other religions were taught well, but no-one was ever told that they SHOULD follow any religion, just what they were, the customs, beliefs etc. Couldn't sex education be like that? It could contain information on homosexuality, trans people, and even pornography too, not advocating it, but discussing it, how it (porn) presents men and women; how it is not necessarily realistic; the effects it can have on people. Again, as someone said above, MANY 12 year olds will have seen porn, either by chance, by choice or as part of abuse. Wouldn't it help if children had had a chance to discuss it with their class, especially if they are being abused?