The document reiterates church teaching that gay unions are not equal to traditional marriages
Pope Francis says Catholics should use their conscience - not strict rules - when deciding on their attitude to sex and marriage.
But in new Church guidelines on family life, the Pontiff's approach to homosexuality remains the same.
In a 256-page document called The Joy of Love, the Pope strongly reiterates the Church's opposition to the legal recognition of gay relationships.
Elsewhere, Francis makes clear he wants nothing short of a revolution in the way priests deal with followers.
He says the church must no longer sit in judgement and "throw stones" against those who fail to live up to the ideals of marriage and family life.
Editor of the Irish Catholic, Michael Kelly, says there is a revolution in language used in the letter.
"He's not changing any of the teaching, but he's saying language needs to change and the approach to people needs to change," Mr Kelly explained.
The document's release marks the culmination of a divisive two-year consultation of ordinary Catholics and the church hierarchy.
Pope Francis slaps down proposals to place gay unions on the same level as marriage, saying bishops found "there are absolutely no grounds for considering homosexual unions to be in any way similar or even remotely analogous to God's plan for marriage and family".
He also slams external pressure on churches to change their position and says it is "unacceptable" that "international bodies should make financial aid to poor countries dependent on the introduction of laws to establish 'marriage' between persons of the same sex".
The Pope offers sympathy to those families with gay relatives - "a situation not easy for parents or for children" - and says the Church must avoid "every sign of unjust discrimination" towards homosexuals.
The pontiff says couples who live together outside of marriage "need to be welcomed and guided, patiently and discreetly", and the choice to cohabit may be based on external factors such as financial difficulties or cultural situations.
Some of the other topics covered include:
"They are not excommunicated and should not be treated as such", and should be made to feel part of the Church "while avoiding any occasion of scandal". The Pope says the Christian community caring for such people "is not to be considered a weakening of its faith" but a sign of "its charity".
Francis says pastors should judge situations on a case-by-case basis. "We know that no 'easy recipes' exist," he writes.
Offspring should be taught to say "Please", "Thank you" and "Sorry". They should be punished for misbehaviour, cured of the vice of "wanting it all now" and prevented from watching TV programmes which undercut family values.
The Catholic Church
"We need a healthy dose of self-criticism", says Francis, admitting that until now the Church has "proposed a far too abstract and almost artificial theological ideal of marriage" and struggles to present marriage as more than "a lifelong burden".