Priests are privy to people's innermost confessions. They receive information that is often not transmitted even between spouses. They can provide the kind of insight which can only come from one who is an outsider to the relationship. Whether that advice comes from one who is married or not is irrelevant. Who would you rather receive marital advice from: Pope John Paul II, who is celibate, or Bill Clinton, who as we all know is married?
Furthermore, do you ever advise anyone? Using your standard you can never really do so, as you will never experience what it is to be subjectively in their shoes.
You've got to have some sort of referential experience in order to give useful advice. This is one of the big lessons we learned during and after Viet Nam. We had all these vets coming back with emotional problems because of their war experiences and they went to shrinks that turned out to have dodged the draft and had no idea what is was like to slog through the jungles with people shooting at you. And they couldn't relate. The vet would talk about his experience and the shrink would say they understood and everybody in the room knew the shrink was full of crap because he had no way to understand.
It's not the priests fault and I'm not attacking them. I am attacking the Church that said for a number of years that you shouldn't seek advice from anyone other than your priest. They put priests in a position where they had to deal with things they couldn't. If you want to know what the Bible says about something, ask a priest. If you want to know how to solve difficulties between yourself and your spouse, ask somebody who has at least tried to solve difficulties between themself and their spouse.
I do give advice, I give a lot of advice, it's my thing. But I only give advice on things I understand, things where I have parallel experiences. Anything outside of that realm I'm completely honest with my friend and tell them I'll listen all they want but I won't give advice because I'd just be making things up off the top of my head with no foundation. And I think so called marriage counselors that have never been married are a joke and people that go to them are fools. It's like asking someone that's never seen a car before how to rebuild your engine, how would they know?