The reform would grant full marital rights -- including the possibility to jointly adopt children -- to gay and lesbian couples.
"The path to equality is open," said the chairwoman, Renate Kuenast of the left-leaning ecologist Greens opposition party, in a tweet on Wednesday.
The bill is widely expected to pass as it is backed by most parties and Chancellor Angela Merkel has told lawmakers of her centre-right party that they can vote according to their conscience.
The reform would grant full marital rights -- including the possibility to jointly adopt children -- to gay and lesbian couples, who in Germany are now only able to enter so-called civil unions.
The bill was hastily put on the parliamentary agenda on the last day before the summer break by the centre-left Social Democrats, Greens and far-left Linke party.
They acted after Merkel had on Monday night dropped her long-time opposition to the reform, sparking widespread calls for a speedy vote.
Merkel had long voiced personal reservations about gay marriage with adoption rights, citing concern about "the well-being of the children".
Last Sunday, her junior coalition partners and election rivals, the Social Democrats (SPD), upped the ante by declaring they would insist on same-sex marriage in any future alliance.
All other political parties hold the same view, leaving Merkel's party opposed along with the hard-right Alternative for Germany (AfD).
Merkel signalled her shifting position Monday in an on-stage interview with the editor of women's magazine Brigitte.
She said her thinking had changed after a "memorable experience" when she recently met a lesbian couple who lovingly care for eight foster children in her Baltic coast constituency.