lol, many different type schengen visa not only tourist visa.
And OP asked for how to prove his relationship. If you read my comment further down, I responded. She need to show strong ties to return to Thailand, many pictures, chat log, call log and more.
If you apply non-o based on marriage visa before you go Thailand then no problem with airlines but if you come on tourist visa or visa extempt then expect you need return flight or onward ticket depends on your airline.