Rhaenyra was going to be put in danger regardless of who Viserys married. Viserys should not have remarried and had sons if he didn’t want Rhaenyra’s position to be threatened. Westerosi custom is clear about sons before daughters and it’s common sense that there will be war if he keeps Rhaenyra as heir while having sons. Corlys Velaryon was planning the exact same thing when he proposed that Viserys marry Laena and Rhaenys told Rhaenyra as much. As long as Viserys had sons, they would always be a threat to Rhaenyra’s position, which is why Otto claims that Alicent’s children would have to die for Rhaenyra’s position to be secure.

In an ideal world, Alicent could have just refused to marry the king and refused to have sex with him. It’s not that kind of world, though, and she was trying to be loyal to Rhaenyra and trying to ignore what it meant for her own children, but Rhaenyra lying to her, her defending Rhaenyra thinking she was telling the truth, and then her father being sent away partially on account of her believing her and thus leaving her in King’s Landing without her only ally as well as the realization that her children will probably have to die for Rhaenyra to thrive was probably the “turning point” for her.

I don’t understand why Viserys couldn’t have just had a mistress. His decision to remarry and have more children was stupid. He could have just taken Alicent as a mistress (as gross as that also is).

In real life, the Anarchy wasn’t between a daughter and a son but instead a daughter (Henry I’s only son died) and a nephew. This plot (and war) basically relies on Viserys being a moron.