This has nothing to do with politics or with the neighborhood stories. It's been like this since game release -- my guess is it's an issue with the game engine and multitasking and the emotion system (which is shallow and lackluster). Sims 3 was set up where once a sexual preference was set, it stuck and it would take a lot of work to get a sim to switch teams so to speak, which also could be a bit of an issue. You could use mods to refine the system (NRaas SP was particularly robust when it came to setting sexuality, but a system was already in place).
Really, ideally you would be able to set sexuality, including monogamy, through neighborhood stories. I'd also like for group chats to be a lot less invasive. That is actually one of my biggest complaints about Sims 4 -- everyone is way too chatty, and there are a lot of times where the chatting is completely inappropriate. I'd prefer it if group chats involving an active sim had to actually be initiated by the active sim.
