Parabatai are not allowed to fall inlove right? What if the other one became a Downworlder (Like the case of Luke, he became a werewolf)? Can they still be together?  — Louisdaniele

Could Luke and Valentine still be together? Well, there is much that stands between them. The turbulent sea, their vaunted heterosexuality, the fact that Valentine tried to kill Luke — but in love, there is no such thing as a truly insurmountable obstacle.

Parabatai can’t be together romantically under the Law (if one of them falls for the other but not vice versa, that doesn’t matter. Pining is free.). Unless they want to quit being Shadowhunters and become mundanes. Then they can be together. Shadowhunters can’t be with (by which I mean marry: casual relationships get a pass in the modern age) Downworlders or mundanes either, unless they become mundanes (Will’s parents). If you want to do something against the Law you can always become a mundane because then the Law doesn’t apply to you. There is no point becoming a Downworlder to be with your parabatai romantically because then one of you will still have to become a mundane which was already an option you could have chosen.