It depends on what is yous purpose ? If you want to evaluate their knowings, a questionnary will be good enough. If you want to know much deeper what they beleive, it needs face to face interviews with open guides. If your purpose is to give them knowings, I believe the better way should be peer-groups with an expert here to answer the questions raised by the group. But of course this must depends on local culture
I agree with Frederick. But then the most factor is your object and your theme of study. for example in some subjects like Sexual health and family planning you Cant achieve your study goals becuse of cultural condition Especially in rural areas.
It depends on what you plan to achieve and the literacy level of your community of interest. However, face to face is the best. Note:Some cultures do not discuss sexuality openly, if at all.