Two debates have crossed here. One is whether or not pointing out somebody's perversion or immorality constitutes "judgment" or "discernment"; the second is whether Orthodox Christian norms of behavior may be applied to those outside the Church. Regarding the former, it's hard to say what the OP intended by using terms like "pervert", but I see no reason to think he was not simply discerning this man's objectively perverse behavior (when measured against traditional norms), rather than arrogating to himself judgment over the man's spiritual status, which is God's prerogative. In order to determine whether or not he was judging in the latter sense, I think we would need to be able to see into his heart, but we cannot.
Regarding the latter, you sort of made my point again. Whether or not this man is "perverted" as opposed to merely "immoral" depends on the highly fluid norms of contemporary society. I wouldn't be surprised if the OP grew up to think of this behavior as perverted, while the other man in question grew up to think of it as immoral at best. I think it suffices to say that according to Orthodox norms, this behavior is perverted, and that on an Orthodox forum the OP is allowed to assume that others share the same assumptions. I'm not aware that the Church applies different moral norms to those outside the Church. What is certainly true is that those who break God's law in ignorance will be judged more leniently than those who do so in knowledge of the law, but that is a matter of judgment in the second sense, as I outlined above.
In a personal opinion though, after re-reading the OP, I think the guy is a pervert. Any time you sleep with a mom and then her daughter ESPECIALLY when you are married to the mom and it is your step daughter - that sounds pretty perverted to me. I don't care if she was over 18.
I would classify this as an perverted.