I read from other sites that one should not have sex before being treated with acupuncture - why is that? How long before does it affect the treatment?
What you have heard is correct, but it is not just sex that should be avoided before (and after) acupuncture. People should avoid high states of emotions, heavy meals, alcohol, stimulants, etc. Basically anything that strongly effects a persons energy balance. Probably best to avoid 3-4 hours before and at least 3-4 hours afterwards. In my opinion it is the afterwards which is more important....
While there are a lot of reasons for this it basically comes down to two main reasons. All of the items listed above create significant changes in a persons energy balance. Eating, for example, draws your circulation into the abdomen so you can digest well. Offering a treatment to someone may disperse the inward energy they are using to digest - as a simple example.
From a diagnostic perspective it can be more difficult for the practitioner to come to a clear Chinese Medicine diagnosis when your energy is not in a more stable state. This can confuse the practitioner and make it more difficult for him/her to come to an accurate diagnosis and treatment plan. Most practitioners know there way around this to some degree, particularly as medications often cause disruptions as well - but there is no benefit in making it more difficult.
From a treatment perspective you can override - essentially negate - the effects of acupuncture by too strongly stimulating your body after a treatment. So in an extremely simplifed example, if you have wrist pain and the needles are used to direct your bodies energy out to the area of pain to work on the process of healing the area - then you eat, have sex, etc. - all of that energy is then redirected to the abdomen, the sex organs, etc. thus disrupting the treatment.