In the eyes of the government of Singapore, which is what we are trying to point out to you, you have not been in a de facto relationship.
First of all, Singapore only recognizes de facto relationships that have been entered into prior to coming to Singapore.
Secondly, as she ONLY has a social visit pass, she has had to leave the country each time, only to return again, The lack of residency also means, in ICA's eyes, no relationship of any sort of permanency is there, therefore, as she is from one of the countries that traditionally have young girls who come to Singapore for immoral reasons, multiple trips means she would have to be working illegally as otherwise she has no means of support.
Thirdly, what you have been given here is exactly what ICA thinks, not our personal beliefs/knowledge. The ICA have a set checklist where people are pigeonholed based on their travel habits against known data. It's called profiling. Nobody likes it, but everybody is affected by it. Get used to it. Happens everywhere today. If you fit the profile and don't have legal documentation to prove otherwise, you are up the creek without the paddle.
What beppi was saying, is exactly what the Immigration officer thought and that's why your gf was yanked from the line. 6 visit visas in 6 months without viable means of support, well, looks pretty obvious when the vast majority of lone girls, from certain Asian countries, make multiple trips here.
Honestly, I don't find it upsetting that providing physical pleasures in return for food, board and occasional travel is not on the list of activities that gets somebody a visa.
Oh, one last thing, I've never met beppi.