If Cephas was allowed a wife, why not Paul (Mk.1:30)?
We don't know if St. Paul was "not allowed" to marry. It appears from his letters that he simply chose to be celibate. Like I said, the celibacy discipline was not adopted legally till the 4th century, and many Christian clergy were adult converts already married; such was St. Peter.
This being said, the Church always distinguished between St. Peter's situation,-- a married man entering priesthood (allowed in the early Church, still allowed in the East, and for converts in the West), -- and what would have been the case for St. Paul, an ordained clergy marrying (not allowed anytime and anywhere, as far as I know).