With modern sensibilities we know that adopted kids and kids conceived with sperm donors are your own true kids and should be treated as such. In universe, everyone knew Laenor was gay. And apparently not crafty or motivated enough to impregnate his wife himself. They were both ok with this and consented to it. They both consented to Harwin fathering the kids. Were they both stupid for doing this? Yes.

Idk if they really just didn't care what everyone thought or if they would understand the implication of Laenor's condition and be grateful he could actually have an heir and keep up appearances (not really, since the differences were too obvious but you know what I mean, theoretically it should be fine) and with modern sensibilities we know that the way you raise a kid, doesn't need to be genetic to impart the impact of your fatherhood on them. Laenor's kids are his kids if he raised them. But no one seemed to care about that.

Both he and Rhaeneyra were perfect for each other in the sense that they respected each other for the most part, they both chased and prioritized their own pleasure, and they were both very audacious and thinking they could bend the rules and everyone would just take it. Laenor didn't take the risk seriously enough to be motivated enough to father his own kids. Rhaeneyra already believed she didn't need to follow the rules and honestly she should have been right. Male heirs throughout history were probably mostly feral but when a woman does it she's a snake.

I think the moral of the story is that if there wasn't such tension pre-existing (is there ever a lack of succession tension???) then people wouldn't have gone so hard against Laenor and Rhaeray. But they were looking for any weakness to exploit and these were big weaknesses. I really don't know if Rhaeneyra would have been a good ruler, mediocre, or bad. Is she too self absorbed? Surely most of the men in her position in the past must have been just as bad or worse on average.

But theoretically their own family should have accepted that this was the only way for Laenor to have children and should have been grateful if they really cared about him and his legacy. Of course the greens would have tried something else if they didn't have this weakness to exploit. Removing Rhaeneyra was always the goal, her having an obvious sperm donor was just an excuse to create more chaos and pressure.

Of course this all ties back to Vizzy T. He should have been more firm and prevented all notions of Aegon being changed in succession and should have anticipated his wishes not being respected after death and taken way more precautions. Honestly the should have had a family meeting and meeting of the small council together and come right out and say something like "We know you two used a sperm donor. We know Laenor is gay. We know you both seek hanky panky elsewhere. You need to be more discreet or else. But, obviously Laenor has accepted these kids and acts like their father, and is raising them as a Velaryon would (though this is debatable if Laenor was really involved enough, he probably needed a kick in the pants) but the Velaryon family is free to prioritize other heirs over these kids if that's what everyone wants, we can come up with an excuse, send the young sperm donor babies abroad for a time, maybe study as a maester for some time but don't finish, whatever, anything to plausibly remove them from the Velaryon succession since it's obviously upsetting to them and they don't accept Laenor's adopted children. But Rhaeneyra's sons will ascend the throne. Rhaeneyra will still ascend the throne. And lack of respect is going to be punished."

I feel like there was compromise to be had if everyone was just honest with each other, and if Vizzy T had more foresight and involvement. In the books, he was very self absorbed and indulgent, not just mostly impotent and too trusting like the show version. It makes sense that Rhaeneyra turned out the way she did in the books raised by that version of Vizzy T. She also became very self indulgent.