I would like to know why Catholic priests believe they may give us married couples advice if they are not married.
Because it is a question of doctrinal, moral, and pastoral preparation. Jesus Christ, while celibate, also legislated on marriage (cf. Mt. 19). And St. Paul, being celibate and recommending consecrated virginity, did not hesitate to write to married couples:
“Now concerning the matters about which you wrote. It is well for a man not to touch a woman. But because of the temptation to immorality, each man should have his own wife and each woman her own husband. The husband should give to his wife her conjugal rights, and likewise the wife to her husband. For the wife does not rule over her own body, but the husband does; likewise the husband does not rule over his own body, but the wife does. Do not refuse one another except perhaps by agreement for a season, that you may devote yourselves to prayer; but then come together again, lest Satan tempt you through lack of self-control. I say this by way of concession, not of command. I wish that all were as I myself am. But each has his own special gift from God, one of one kind and one of another.
To the unmarried and the widows I say that it is well for them to remain single as I do. But if they cannot exercise self-control, they should marry. For it is better to marry than to be aflame with passion.
To the married I give charge, not I but the Lord, that the wife should not separate from her husband (but if she does, let her remain single or else be reconciled to her husband)—and that the husband should not divorce his wife.
To the rest I say, not the Lord, that if any brother has a wife who is an unbeliever, and she consents to live with him, he should not divorce her. If any woman has a husband who is an unbeliever, and he consents to live with her, she should not divorce him. For the unbelieving husband is consecrated through his wife, and the unbelieving wife is consecrated through her husband. Otherwise, your children would be unclean, but as it is they are holy. But if the unbelieving partner desires to separate, let it be so; in such a case the brother or sister is not bound. For God has called us to peace. Wife, how do you know whether you will save your husband? Husband, how do you know whether you will save your wife?
Only, let every one lead the life which the Lord has assigned to him, and in which God has called him. This is my rule in all the churches.” (1 Cor 7:1-17)
Some might say: “But Jesus Christ is God, and St. Paul was an apostle.” As a decisive argument it carries no weight; however, in order to put the questioner’s mind at rest, I could remind him that in addition to the examples of Our Lord and of the Apostle, St. Paul himself commands a priest and bishop, who was celibate, Timothy, to give advice and to direct the elderly, youth, mothers, widows, etc. (cf. 1 Tim 5:1ff); and he tells him very clearly “Command this, so that they may be without reproach.” (1 Tim 5:7).
He also commands another of his disciples, Titus: “But as for you, teach what befits sound doctrine. Bid the older men be temperate, serious, sensible, sound in faith, in love, and in steadfastness. Bid the older women likewise to be reverent in behavior, not to be slanderers or slaves to drink; they are to teach what is good, and so train the young women to love their husbands and children, to be sensible, chaste, domestic, kind, and submissive to their husbands, that the word of God may not be discredited. Likewise urge the younger men to control themselves. Show yourself in all respects a model of good deeds, and in your teaching show integrity, gravity, and sound speech that cannot be censured, so that an opponent may be put to shame, having nothing evil to say of us. Bid slaves to be submissive to their masters and to give satisfaction in every respect; they are not to be refractory, nor to pilfer, but to show entire and true fidelity, so that in everything they may adorn the doctrine of God our Savior.” (Titus 2:1-10).
Evidently, we can here see something of common sense: when we want a doctor to cure or counsel us, we do not search for a doctor that is sick like us but rather one that knows, even if not from experience, how to treat our sickness. Although miles apart (matrimony is not a sickness but a sacrament!) we can glimpse the analogical application to our case.
Original Post: Here
Other Post: Can absolution be denied to someone who is not married in the Church?