When my son was twelve, his father and I divorced. After which the two of us became pretty close and felt quite relaxed and comfortable in our mother-son relationship. Feeling that it was permissable for him to occasionally see me undressed, and that there should be no big mystery about female anatomy, I often went between bedroom and batheroom without feeling the need to bother covering up. Especially in the mornings in the usual rush to get ready for work. My son was use to this sort of unclothed casualness, although I rarely saw him with nothing on, as he shyly preferred to wrap a towel around himself.
I was admittedly somewhat surprised after he turned 14, when he told me that he liked seeing me nude and asked if we could be nude together sometime so he could see what that was like. With the assurance that things like family nudity was something private and to be dsicerete about, I agreed. So quite casually we went ahead and tried having our clothes off and being nude together around the apartment. Not unexpectedly my son got an erection and was self-conscious about his arousal. I assured him that it was natural and nothing to be embarrassed about. A situation which allowed us to bring up the subject of sex and to have a relaxed discussion about. Something which was quite pleasant to be able to do so in such an open and honest manner.
On the next few occasions when we happened to be nude together we continued our discussions, and the topic of masturbation came up. While I was fairly certain that he was already doing this, and knowing that it was a pasttime that I indulged in, I assured him that masturbation was a natural and normal form of safe sexuality. And, yes, one that grown-ups engaged in, including myself. Since it seemed opportune, with both us sitting there naked, we ended up masturbating in front of each other. A uniquely satisfying and pleasurable experience, and one which we began to regularly practice together.
At one point, just naturally curious, my son asked if he could try having intercourse with me. Although this would have been quite easy for me to allow, I explained to him the risk of his fertilizing me and getting his mother pregnant as a result. Which in turn led to a very good and straightforward discussion on birth control. After which he never asked to have sex with again and developed a mature attitude towards copulation and all of the consequences that it could have.
Although I realize that all families are different in how they approach sex and sexuality, I can only say how openness and honesty can play a critical role in dealing with issues of misunderstand and misconceptions.