You can get disfellowshipped for marrying an unbeliever. And it is automatic if the unbeliever is a disfellowshipped witness. I think the branch manual said that if a person would willfully remarry their disfellowshipped former mate, that would be grounds for their disfellowshipping. Someone can "fact check" that but I believe it is one of the only instances where it is automatic disfellowshipping and not automatic disassociation.
Not sure on what exactly there is in print on this matter but I would still suggest that technically it's not the marriage you are getting DF'ed for it's unrepentant and wilful association with a disfellowshipped person.
It's one of those silly WT procedural technicalities that they get themselves wrapped up in because they cannot stop writing pseudo legal interpretations of scripture.