All couples in Scotland should be given the same rights, regardless of gender, said Scottish Government's Health Secretary, Alex Neil.
"We have always maintained at the heart of this issue there is one simple fact: a marriage is about love," he said.
Legislation to allow gay marriage in England and Wales was passed at Westminster last July.
The Marriage And Civil Partnership (Scotland) Bill also takes account of celebrants who do not want to take part in ceremonies, Mr Neil said.
The Bill puts belief celebrants, such as humanists, on the same footing as religious celebrants and allows civil partnerships to change to marriage.
The final debate will be a last chance for opponents to alter the legislation.
SNP backbencher John Mason is attempting to add a "protection" for people who believe marriage is between a man and a woman. His party colleague Richard Lyle wants assurances that opponents of gay marriage will not have their views used against them when trying to adopt or foster children.
Labour MSP Siobhan McMahon is also trying to add protections for "freedom of expression".