It’s important to
remember that Lyanna was not only a kid, but a kid with canonically romantic –
although well-founded – ideals, even if she did have a practical side. People
wonder why she would have run away with Rhaegar, a married man, when she
rightfully excoriated Robert for being unfaithful. It’s a good point, but…

…well, Rhaegar probably
spun a tale of a prophecy that only she could help him fulfill, and that would
have seemed a LOT more romantic to her than the idea of Robert the Canonical Cad, who swaggered around and whose motto can
be best summed up with “If she’s female and she’s breathing, stick it in” (best
sung to the tune of If You’re Happy and You Know It). So she chose what seemed like the only way to get away from him, and Rhaegar sold it to her as ‘well, it’s not really a bad thing at all if you help save the world, wife, what wife?’ She enabled Rhaegar, but
I’m firmly of the idea that it was entirely unintentional, and she’s not to
blame because she was fifteen years old.

On a side note, Lyanna
being upset over Robert siring a lot of known bastards, and wanting him to be
faithful to her, is not dissimilar to what Catelyn feels about Ned having sired
Jon and raised him alongside their kids. Both perspectives are entirely valid, and both should be defended more than they are.