First off, holy orders requires a vow of continence and a marriage cannot be contracted if someone has taken a vow of continence. The marriage contract says that a man and a woman give and accept a perpetual and exclusive right to acts which are, of themselves, suitable for the generation of children. If one has taken a vow of continence, they cannot give or accept rights to acts which are suitable for the generation of children. This is why a married man can become a priest in the Byzantine rite, provided his wife agrees to not ask for the marriage debt. This is also why a priest will never be able to marry.