Why do you say Rhaegar and Lyanna's relationship is abusive? We know next to nothing about them.
With what we do know, there’s no way their relationship could be anything but abusive. Rhaegar was a 24 year old married man with two children and a crown prince of the kingdoms, and Lyanna was 15 year old girl who just wanted to escape her impending marriage. That’s one hell of a power imbalance. Rhaegar may not have hurt her physically (that we know of), but that doesn’t mean he was good to her or for her. Lyanna may have very well gone with Rhaegar willingly, but that does not mean she agreed to be hidden away in a tower and impregnated at 15 to fulfill Rhaegar’s ridiculous prophecy. In all likelihood, Rhaegar didn’t love Lyanna, he was using her as a human incubator for his third head of the dragon.
Going after Lyanna was the worst mistake Rhaegar ever made, and yet D&D are twisting it into some kind of tragic love story to make Jon Snow’s real daddy more likable. Any way you slice it, Lyanna’s death is on Rhaegar. And not just Lyanna’s: Elia’s, Rhaenys and Aegon’s, Arthur Dayne’s - Rhaegar’s own best friend - and thousands more in a completely pointless war that Rhaegar began. He was the heir to the Iron Throne, and Lyanna was just a girl; he was the adult, and she was the child. What happened to Lyanna was Rhaegar’s fault. And that’s not romantic, it’s not tragic - it’s inexcusable.
You say we know next to nothing about Rhaegar and Lyanna, but what we do know works directly against them having a healthy relationship. Lyanna is described by more than one person as being “willful” and having “the wolf’s blood.” She wanted to be free; to live as she choosed; to explore, to fight, to be outside. She was impulsive, headstrong, and independent. Lyanna didn’t want to marry Robert Baratheon because she didn’t want to be trapped with a man she knew would be unfaithful to her. So by what logic would she consent to being confined to a tower under guard for over a year by a man who was unfaithful to his own wife? Isn’t that exactly what Lyanna was trying to avoid by escaping her marriage to Robert?
And then there is Elia Martell, a woman that should not and cannot be ignored just so the R+L ship can sail. Abandoning your sick wife and young children for a teenage girl is not romantic, and yet here everyone is, romanticizing it. Did Robert’s Rebellion not teach us anything? That actions have consequences? That when you act selfishly and impulsively at the expense of others, retribution will come? If Rhaegar and Lyanna’s “love” was so pure and beautiful and true, why did Westeros break out into a civil war for her honor? Why did both of them die as a result? What was the point?
Book!Rhaegar believed in something, at least. Even though he was wrong, he still truly believed that what he was doing was for the good of the realm - to save the world - which still doesn’t excuse his behavior. But show!Rhaegar? Prophecy who? He had no reason to steal Lyanna except the fact that he wanted her, fuck everyone else who got in the way, right? But it’s okay, because together they made Jon, right? Wrong. Jon Snow is a very important man, but his existence doesn’t somehow validate Rhaegar’s actions. “Who cares what Rhaegar did! Without him, we wouldn’t have Jon!” But what about Lyanna? What about Elia and her children? What about the realm? Is your favorite broody boy really so important to you that you’re willing to overlook every horrible thing that happened to give him to you?
I don’t hate Jon, I love him, just as I love Lyanna. She deserved far better, and so did Elia, Rhaenys, Aegon, Arthur, Rickard, Brandon, and everyone else who died because of what Rhaegar Targaryen did. Why do I think R+L is abusive? Because Rhaegar destroyed Lyanna, and you don’t destroy the person that you love.