You are correct, threaded couplings are designed to make fluid (gas or liquid) tight connections. The original design was conceived with the intention of supporting the connection, the pipe and the contents of the pipe with a reasonable
safety factor and not leaking. They were never intended to be structural fastenings carrying lateral and vertical loads as a part of a load bearing structure. IMHO, the only acceptable way of using a threaded coupling under load, is in direct compression such as a pipe light
boom threaded into a 50#
boom base, and with lateral and diagonal bracing as needed. Other acceptable uses of threaded couplings are as a positioning device, such as a wall flange to keep a pipe from swaying when it is supported by other means. They are OK for decorative structures that are not load bearing and for light weight furniture. Even in those cases, the structure needs to be designed and built to minimize points where a threaded portion of the pipe is carrying
load in a shear or lateral bending condition.
Threaded couplings in compression loading, strongest condition. In straight tension, second strongest, not to be used for
overhead lifting. Any condition where the coupling supports more than the weight of the pipe and any contents in a shear or bending condition, to be avoided at all costs. HTH.