Please answer all the questions you wish, and skip the ones you don't want to answer, for whatever reason. Some of the questions allow multiple answers, and all of them have a field for extra comments, which are welcome even if you haven't answered the question.
Incidentally, the original questionnaire had a question 20 - 'Can you draw a diagram of your relationships? Please indicate what words/lines/notation you have used (e.g. dotted lines for sex friends, wavy line for partners' partners)'. This has been omitted from the form for obvious reasons, but if you have such a diagram in e-mail-friendly format, feel free to e-mail it to marcus@adjectivenoun.org.uk.