Since sin is sin as you say and all is covered by Grace then it really doesn't make any difference what the sin is. You say, " why Nikos, don't you know that there are all kinds of people that have ministries, eg. ex-porn and ex-murderers, etc., so what is the difference!
Now if you are correct, then we should be able to bring up any sin and if the person is saved it would not make any diifference to you.
Now I am first to agree that Grace can save anyone. But there are sins that are severe to the point of disqualifying ones self from ministry leadership.
Suppose your pastor pre conversion regularly exhumed corpses from the graveyard and sexually violated them. Or suppose he kidnapped small children, starved them, maimed them, and made them believe that their parents were dead. Or suppose he framed minority men for crimes he had committed himself so that they would spend years in prison away from their families. Would you ever want such a man pastoring your church with youth and children?
I don't think you would. But not because you doubt the power of God's Grace. For we agree, God's Grace can redeem reprobate sinners whom we regard as hopeless. But while evil people of darkness can be miraculously saved, we still don't want them tucking in our kids at night.
Now these sins are horrible and scary,( I tried to think of the worst sins, like ISIS, to show that it is the severity of sin that is the issue and not the Grace of God) yet I think that adultery ranks right up there. All we have from TB is his testimony and his life, which we can only see a part of, the public part.
All of these sins can be forgiven. But how can we be sure of this man's relationship to God. Adultery is a terrible sin. He needs to sit down and let others lead. He can find a supporting place in the pew.