It is my business when it affects the attitudes towards foreigners around me. Unfortunately I along with expats get painted with the same brush whenever the Chinese see someone behaving badly. It seems there are those that come here and all social constraints are lifted allowing them to take advantage of the situation. no doubt they are scumbags in their own country but do not out there because they would be knocked out or jailed.
I personally do not care if someone has as much sex as they want but I do question those that use their position in the schools to pick up young girls that have not had dealings with men. with no regard for their future many of these foreign scumbags take advantage of the girls then leave them by the roadside after they have had their fun. An American here had his way with a girl who is clearly mentally unstable and afterwards pretended that he had done no wrong. i am sure in every city there are stories of scumbags who prey upon these woman and yet the foreign community says nothing. By ignoring this behavior the foreigners are saying it is alright to do whatever you want for there are no real consequences. If I was not affected by others actions I would have nothing to say but that is not the case and so I choose to say what I will.
if someone chooses to get drunk and whore around all the power to them as long as they do it responsibly. I really do not care what most people do but if someone does do something I do not agree with I will add my 2 cents worth. As much as I cannot stop them from doing what they want they cannot stop me from saying what I want.

What foreigners do in China in absolutely none of your business

It is if i have to deal with the repercussions of their behavior after wards. It almost sounds like you are forgiving rape, pedophilia and other sundry actions. If you ran into a guy diddling little kids you would say nothing?