It's easy. Just don't do stuff that can get you shadowbanned. /s
A more serious answer, you're unlikely to get shadowbanned for doing marriages impossible in the base game via the Gay Fates Hack - the creator of it never ran into any problems online.
You're more likely to get shadowbanned if you mess around with save editing stuff. Iirc you can get shadowbanned stuff like enemy only skills, enemy only classes, more units in Nohr Prince(ss)/Kitsune/Wolfskin/promos of them than you are allowed, enemy only weapons, et cetera. Just keep a backup of your non-edited saves and restore them if you get banned, you'll be fine. You also won't get banned for something like Dark Knight!Saizo because he can get Dark Knight if MU has that as a talent and Saizo has an S or A+ with MU.