"The Truth" compels marriage within.
Its members would like to disagree. But that's a ruse. The ruse is not necessarily intentional or even conscious; but it's pervasive.
Extra-denominational fraternizing is prohibited. The sect is endogamous, absolutely, with recruitment the sole approved vector to intimacy outside of the church.
There are workarounds, because they're necessary, but the code is strict.
Even so, the rule itself is never specified. And this is what allows the central ruse, which is that we never said that.