My thoughts exactly..
Testing the.. fertility of its soldiers seems like something the BoS would do anyways, since they prefer to have their members born and raised brotherhood rather then recruited. At least thats how it is in the Mojave chapter. Procreation is considered one of the duties each brotherhood member should contribute to, and was the preferable means to acquiring more soldiers, since the brotherhood had one of least mutated bloodlines in the wasteland.
I tend to think *some* of that methodology lingers even in Maxons chapter, as some of the PA intercoms prohibit fraternizing with the locals.
If the synths truly are indistinguishable from human beings through all forms except dissection, then all physical tests would indicate the Synth was a human being to the commonwealth. Unless they simple are created sterile, which if found out to be a distinguishable trait, wouldn't be too hard of test to reproduce for finding the potential synths among the populace. If they can reproduce with human beings, it becomes a much more difficult subject in regards to morality to deal with. It would really swing either way in regards to wether people thought they had the rights of human beings, or if- for being able to create human lives, but not be human- even more abominable.