many of these people literally have sex it just doesn't count for them because it wasn't under a socially enforceable permanent basis like a traditional patriarchal marriage

As someone who has spent a lot of time studying one of the larger incel communities and its offshoots, this is false. Several of them have had sex before but have no access to it anymore, and some might not count paying a prostitute, but a casual hookup or a girlfriend absolutely do count to the vast majority of them and most of them pine after it, though of course most have as their "ultimate" desire to have a wife for the reasons you describe.