The Archbishop of Lingayan-Dagupan, Socrates Villegas, told Sun Star Manila same-sex unions could never be a marriage.
The President of the Catholic Bishop's Conference of the Philippines also made it clear family and friends of gay people are to love them "unconditionally", without actually consenting to gay relationships.
Same-sex marriage is currently illegal in the Philippines.
A petition is due before the country's Supreme Court, arguing that banning gay couples from marrying is unconstitutional because it denies them equal rights with straight couples.
Archbishop Villegas told the Sun Star Manila: "Catholics cannot participate in any way or even attend religious or legal ceremonies that celebrate and legitimize homosexual unions.
"They are not able to contribute in a proper way to the procreation and survival of the human race, and thus it would be an injustice to grant them legal recognition along with the same benefits and privileges accorded to marriage.
"Families with members, who struggle with homosexuality, are called to love them unconditionally, thereby outlasting all their other same-sex loves.
"This love, however, must be a love in truth that avoids praising, consenting to, or defending the so-called 'homosexual lifestyle."
Around half of the Philippines 98 million people are Catholics, with other Christian denominations, Muslims and others making up the rest.
