Legally, the otherworlders exist in a sort of nebulous position where their presence is acknowledged, but not yet given legal status. Which is a long way of saying, that while the pokemon world has adjusted to their presence enough to allow them to enter in work contracts and property ownership, technically, they are not citizens of their world. That would after all be a crazy bureaucratic mess trying to assign citizenship to people who suddenly appear and might be gone again in a flash and then possibly return again.

SO. Legally, Jack and Steven would not be able to get married. Not because they are both men, but because by law, neither of them technically exists. Of course, while that limits their ability to legally wed (on the flip side, no taxes!) that doesn't mean they cannot perform the ceremony socially or indicate to Team Rocket that they are now wed. Finding someone to say some vows and conduct a ceremony is still totally on the table, just the legal aspect that is lacking. (but again, no taxes!)