From my admittedly limited personal experience, I have known 2 British male Marions and around a dozen British female Marians. I was told by the males that the 'o' version is masculine and the 'a' version feminine. Doubtful, as I have read of female Marions, though not male Marians.they might also be surprised to find themselves used as examples of mediaeval naming. they are also all Americans, and with the greatest will in the world, American naming conventions over the last century have somewhat...deviated...from common English practice, viz gender of names and use of very rare names. Marion may be a fine American name and I respect that, but it's not a strong choice for a European king.