I wasn't sure where to look for your questionnaire on the net, if you can provide the web address that would be helpful. You are interested in investigating whether sexuality is an independent dimension or is correlated with autonomy/relatedness in couples. You may well be familiar with David Schnarch's book titled "Constructing the Sexual Crucible: An Integration of Sexual and Marital Therapy" (c. 1991 W.W. Norton & Company. The book integrates both the clinical and theoretical components of sexual intimacy within marriage and seems very relevent to your research. I wish I could do it justice by describing in more detail, but the author does describe an interpersonal state of intimacy in a way that no one else seems to be able to do.
I don't have any thoughts on recruiting subjects. I will however check with Behavior OnLine's editor to see whether we can draw in some other experts in this area for comments.