Psychological effects of social nudity
Quoting from one paper, which doesn't say all that much:


  The experience of nakedness under the gaze of others is highly diverse, and
  important background differences will affect the relative comfort with which one
  might strip in front of others.


Cover, R. (2003). The naked subject: Nudity, context and sexualization in contemporary culture. Body &amp; Society, 9(3), 53-72. DOI: 10.1177/1357034X030093004

And from one of the few studies of nudists in the US, back when this was more common over there (that is 1964):


  Among changes reported by nudists were “less concern
  with appearance,” “reduced anxiety,” “more benevolence,” and “less inhibition.”


Casler, L. (2010). Some Sociopsychological Observations in a Nudist Camp: A Preliminary Study, The Journal of Social Psychology, 64:2, 307-323

Fairly similar findings from a nude group therapy "marathon" (24hrs session):


  Nudity apparently facilitates group interaction in a marathon. 17 of 20 participants felt that the factor of nudity increased their ability to open up to each other emotionally and to achieve a greater degree of authenticity and transparency. The group integrated and seemed to become therapeutically functional more rapidly than clothed marathon groups


Bindrim, P. (1968). A report on a nude marathon: The effect of physical nudity upon the practice of interaction in the marathon group. Psychotherapy: Theory, Research &amp; Practice, 5(3), 180-188

And a pretty interesting discussion contrasting the German and UK cultural differences in how sexuality relates to social nudity is found in 
Smith, G., &amp; King, M. (2009). Naturism and sexuality: Broadening our approach to sexual wellbeing. Health &amp; place, 15(2), 439-446.