"What qualifies someone who isn't married to give advice to soneone who is?"
Based upon this same "logic," why should children listen to their parents? They have never been parents before and are "practicing" on their first-born. Why should the first-born listen to them?
The reason they are qualified is that the Church is wiser than any individuals. Based upon the current societal demise, I would think it should be obvious that whatever it is that parents are attempting to to do, (Oh, those who stay married) is simply not working.
Read what the Church teaches and apply it. For most, cutting off God's grace (through use of contraception) is probably the primary reason why things aren't working.
"And again, other christian religions allow the church leaders to marry. They live their lives, they raise families and still serve God."
And their divorce rates are HIGHER than the general population. And how is it exactly that they are serving God? By teaching heresy to their congregations in a schismatic "faith community"? Come again?
You got some statistical evidence for that? I've never read that the incidence of divorce for Protestant ministers is higher than for the population in general.