The clearest example of this is the persistence of many in believing that smoking (tobacco!) won't do them any harm because 'Aunt Mabel smokes a 100 a day and has lived to 97. In the real world there will always be 'outliers' that occur due to a particular set of unusual circumstances. In theory in order to establish fully the general effect of a particular behaviour it is necessary to do a full 'double blind' trial where otherwise identical twins bought up in precisely the same circumstances are exposed to only the effect being tested for, with the other receiving an experience that seems similar but excludes the reality. (In drug trials this is achieved by giving one subject a sugar pill whilst the other gets a pill with the drug).

Of course in the social sciences you can never get anywhere close to this degree of rigour, so the best you can do is work with as large data sets to establish what you can from many examples.

The alternative way for Christians to come to a conclusion in this area is to accept what the church has always understood to be the teaching of the bible. This is based on the idea that the all knowing God has a good idea what is really the best for us, and has been loving enough to tell us in the Bible. We already accept the bible's authority in some areas of our life - to do so is the definition of being a Christian. The only question is whether we are willing to accept the idea that God has revealed the best for His children in this area - or conclude that some apparent counter examples (without clear evidence about what would have happened without the sexual component to the relationship) prove that a sexualised relationship is a better way forward. (And note of course the BIG interest that we have a BLs to believe the latter...)