"If so that, then what about the others? Ho, ho! Then this so sweet maid is a polyandrist, and me, with my poor wife dead to me, but alive by Church's law, though no wits, all gone—even I, who am faithful husband to this now-no-wife, am bigamist." (13.85)
If Arthur "married" Lucy by giving her his blood, then she's married to four people. Van Helsing finds this hilarious, especially because he's already married, and his wife is insane. Wait, why is that funny?