Technically, in the patriarchal feudal society that is Westeros, Lyanna's permission is irrelevant. Rhaegar took Lyanna without Rickard's permission (as far as we've been told in the books). That's all that matters - regardless of Lyanna's feelings, whether she went willingly or not, without Rickard's permission it IS, legally, technically, kidnapping and rape should he choose to pursue it as such. Which he did.