"Don't canonize me yet," St. Francis of Assisi told his admirers. "I'm perfectly capable of fathering a child."
I don't know whether they believed him, but sexual immorality has always been high on the list of things that interfere with an apostolate. My guess is, if you asked people why this is the case, overall they'd overstate the importance of how much humans love sex and understate the importance of how much we love truth.
A Christian apostle is a sign of Christ, the One Who sends the apostle on his mission. This is true whether or not the apostle wants it to be, whether or not he even realizes it's true.
Humans love truth, so we hate falsehood, so we hate someone who represents himself as a sign of Christ but whose life does not signify Him. Christians, perhaps, hate such false signs all the more in that they are false, not merely to someone admirable, but to Truth Himself.
One conversation that seems to occur whenever a new scandal arises involves the charge of hypocrisy. "What a hypocrite!" some say, while others tease at the definition of the term to see whether it applies in this case. Let me suggest that the sincere charges of hypocrisy indicate, not the misapplication of a specific term, but the imprecisely expressed recognition of this failure to signify what one ought to signify. Saying, "But he isn't a hypocrite as such," is really beside the point when no one really means he is a hypocrite as such.
If moral scandal -- the turning away from Christ caused by another's sin -- that comes with the tabloid scandal -- public reports of the sins of a Christian apostle -- is motivated by hatred of falsehood, anyone involved in preaching the Gospel ought to make clear that he himself recognizes he is to some extent a false sign of Christ, that for example he is perfectly capable of fathering a child. The apostle necessarily signifies Christ; his choice is whether to be an imperfect sign or a false sign.
It's often remarked that, the holier a person becomes, the more aware he is of his own sins. Less often is it remarked that we are aware of how aware the saints are of their own sins. We know this because they have told others of their awareness, and telling others serves not only to instruct us on how sinful we must be, but to make of the saints' lives a true, because admittedly imperfect, sign of Christ.