I read that where Paul said “The Bishop is the be the husband of one wife…” he was saying to obey Roman law which was the law at that time. Was he not speaking against polygomy per se as much as saying to follow the law of the land? Oh course since things have changed and it’s the Bishop is to be the husband of no wife but the church.
NABRE: 1 Timothy 3:1-7
Qualifications of Various Ministers.
1 This saying is trustworthy: whoever aspires to the office of bishop desires a noble task.
2 Therefore, a bishop must be irreproachable, married only once, temperate, self-controlled, decent, hospitable, able to teach,
3 not a drunkard, not aggressive, but gentle, not contentious, not a lover of money.b
4 He must manage his own household well, keeping his children under control with perfect dignity;
5 for if a man does not know how to manage his own household, how can he take care of the church of God?
6 He should not be a recent convert, so that he may not become conceited and thus incur the devil’s punishment.
7 He must also have a good reputation among outsiders, so that he may not fall into disgrace, the devil’s trap.
As I read it this is not a comment on marriage one way or the other. It is a list of qualifications for those who they might appoint as bishops. These include: (NABRE) personal stability and graciousness; talent for teaching (1 Tm 3:2); moderation in habits and temperament (1 Tm 3:3); managerial ability (1 Tm 3:4); and experience in Christian living (1 Tm 3:5–6). Moreover, the candidate’s previous life should provide no grounds for the charge that he did not previously practice what he now preaches.
Timothy 3:  A faithful saying: if a man desire the office of a bishop, he desireth a good work.  It behoveth therefore a bishop to be blameless, the husband of one wife, sober, prudent, of good behaviour, chaste, given to hospitality, a teacher,
This passage has nothing to do with polygamy. In the early Church, many of the men that were called to be Bishops or Priests were married with families. I think what Paul is referring to, is that if anyone feels called to be a Bishop, he should not have any previous wife that he has ‘put away’ through divorce. That’s based on what Jesus taught about divorce being a sin. A Bishop that was married to a new wife, after being divorced, would cause a serious scandal in the Church, so all Bishops needed to be good examples of Christian virtue. It wasn’t until many years later that celibacy was required of the Priesthood.
OK when and why exactly did things change and the Bishops were not allowed to Marry?
I’m not a theologian or a historian, but you can read what it says on the New Advent Catholic website about celibacy. It should answer all of your questions.