Dear, for this class, please read Schalk and Kopf v. Austria (read paras 49-64 and 81-110) - a judgement where the ECtHR refused to recognize the right to same sex marriage (compare with https://en.wikipedia.org/wiki/Obergefell_v._Hodges).  

Regarding positive obligations, please have a look at the Beizaras and Leviskas case (mainly paras 6-24 for the facts and paras 106-129 regarding the arguments. You can always refer to the Guide as a background explanatory material.