What's a just cause for a bishop's resignation?

Like Pax Christi USA, I, too, am sad about the resignation of Los Angeles Auxiliary Bishop and Pax Christi USA President Gabino Zavala. I did not know him, but if he's associated with Pax Christi, he must be in tune with the justice and peace message of the Gospel, and that's great. So why did he resign? He fathered two children.

I'm sad for two reasons. We have not yet seen the day in the Catholic church that a priest or a bishop can marry and father children without it being some sort of scandal. It's long past time that we move to a priesthood that welcomes and celebrates marriage and fatherhood. (And while we're at it, motherhood as well!)

But I'm also sad because resignations are apparently necessary when a bishop "fathers" children, but not when a bishop fails to deal seriously with the abuse of children. I'm thinking of all those bishops who moved abusing priests from parish to parish, who covered up abuse, who have failed to report abuse to authorities. Most of them are still running dioceses.

If we had our priorities straight as a church, which kind of bishop would be forced to resign?