I don't understand it all either, it just seems all so pointless and ridiculous.
I committed the cardinal sin, I married a Catholic...
Ok, then Gay Marriage is not legal, although (and while we are all being pedants here) illegal can be defined simply as forbidden in Law....which conducting a Gay Marriage in Northern Ireland remains...that there is no criminalisation of it through the reason of it's processes simply having no recognition is immaterial.
This 'illegality' is illustrated by your own point that even Same Sex Marriages conducted legally and recognised in other countries both foreign and domestic are also not recognised, and are only given the same rights as Civil Partnerships.
As I said, minds have been made up to paraphrase another forum poster from another thread...