My understanding is that some people read "husband of one wife" as excluding men who have re-married after divorce from being a bishop. But I don't quite understand that, since John 4:17,18 seems to indicate that past marriages are not present marriages, so why is an ex-wife considered a "present" wife?

I agree. I believe a husband of one wife means that he is only married to one woman and that he is faithful to his wife.

I bring up this verse again,
Is it only Pastors that are still under bondage of a marriage when their unbelieving spouse departs?

Justifiable divorce
 

Mat 19:8 He saith unto them, Moses because of the hardness of your hearts suffered you to put away your wives: but from the beginning it was not so.
Mat 19:9 And I say unto you, Whosoever shall put away his wife, except it be for fornication, and shall marry another, committeth adultery: and whoso marrieth her which is put away doth commit adultery.

If a man divorces his wife ( or visa versa) due to infidelity on the others part, the non offending party could re-marry without committing the sin of adultery.

Jesus says "whosoever"...this would include men who would become or are presently Pastors, deacons etc.

I cannot find a reference anywhere in the Bible that disallows men divorced in this circumstance to serve in an office of the church.

Fundy

Yes, I agree.... fornication and abandonment are the two exceptions that result in a Biblical divorce.