I think that is a misinterpretation of the categorical imperative.
A proper interpretation is exemplified by the case of theft. If theft were universal, no one could morally own property since all property would be morally subject to being taken by others. If there is no owned property, there can be no such thing as theft, which is defined as taking without permission that which belongs to someone else. Therefore according to the categorical imperative, theft is self-contradictory.
The celibate priesthood, on the other hand, is not such a case. There is no definitional problem with the priesthood being universally available as an option for all men that satisfy the definitional requirements of the state of being a priest. In fact it already is universal in that sense. What you are probably referring to by your emphasis of the celibate nature of the priesthood is that if all men became priests, there would be no more babies. But that would not result in a contradiction. Furthermore, it would never happen because so few men actually want to become priests. Whereas if theft were made universally acceptable and everyone really believed it, practically everyone would become a thief.
Actually, the priesthood is no different than any other occupation, with respect to the categorical imperative. What about being a butcher? A baker? A candlestick maker? If any one of these occupations became universal in the sense that everyone became that, then there would be no firemen or policemen or teachers or doctors, and society would collapse. I’m sure Kant would not say that being a butcher is contrary to the categorical imperative in the same way that being a thief is contrary. So I think the priesthood is safe too.
I am reminded of the old Sesame Street bit about “You mean you all brought watermelon? Didn’t anyone bring potatoe salad?”. How is that for the categorical imperative?