Clergy and congregations choose whom they marry. They aren't compelled to accept the state's marriage definition, and indeed, many religious institutions don't accept it. Many religious institutions are more restrictive than the state, rejecting interfaith marriages or remarriages after divorce. And some have a broader definition, blessing the unions of same-gender couples. Many religious organizations and people of faith have definitions that are probably different from yours.
Some faiths consider marriage an aid to religious instruction.
Some call it an expression of committed love. Others say it is mainly for raising children. In some Christian faiths marriage is a sacrament, in others it is not. The Universal Fellowship of Metropolitan Community Churches , Ecumenical Catholic Church, Church of God Anonymous , Reconstructionist Judaism , Reform Judaism , and the Unitarian Universalist Association bless same-gender relationships as a matter of policy. The United Church of Christ , and various Quaker groups leave the decision to clergy, congregations or local governing bodies.