Simple, no psychological study can ever be truly considered scientific because for a study in general to be considered scientific, it must be repeatable. Whenever psychological studies in particular are repeated, they usually get wildly different results, simply due to the fact that psychology is based on anecdotes and the plural of anecdote is not data.
Many of the complaints about how Rind et al. is not scientific are complaints that would be fair to make about any psychological study. Psychologists simply ignore those issues for the majority of studies (or they would be out of a job) and point out those issues only when they really really don't like the results of that study.
Additionally Bruce Rind's career has truly been ruined by publishing this study. No other psychologist is going to see what happened to him, copy his study, and risk having to publish results that support Rind et al.