Titus 1:6 and 1_Timothy 3:2,12 --- "One wife" --- mia is the Greek word from which the word, one, was translated in those passages. Yet, it can also be translated as first, just as it is, for example, so translated in the phrases, "first day of the week" in Matthew 28:1, Mark 16:1-2, and Acts 20:7.

Furthermore, in 1_Timothy 5:9, a widow's "one man" is not mia but the Greek word "heis", meaning the numeral-one, and not meaning the adjective of "first".

There is so much more to this particular matter here than that which this "sound-bite" here can address. The organization of TruthBearer.org provides a number of detailed articles on this issue, a couple of which include
Breaking Past the ONE WIFE Barrier
and
The Elders, Bishops, and Deacons Trap
  (Ch.9 of "The Truth and the Paradox").

The fact is, no one can INSIST that these three "one wife" verses can NOT be instead translated as "first wife", which makes more sense to translate those verses as "first wife" anyway.