Tim Hopkins, director of the Equality Network, said its research found that more than half of gay couples who enter a civil partnership complained they were not treated the same as heterosexual married couples.
For that reason, legalising same-sex marriage would "genuinely reduce stigma and discrimination", he told MSPs on Holyrood's Equal Opportunities Committee.
The committee is scrutinising Scottish Government proposals to give gay couples the right to marry.
As reported in The Herald last week, the bill is to be fast-tracked through Holyrood, with MSPs holding double sessions, the first of which started yesterday.
Ministers published the Marriage and Civil Partnership (Scotland) Bill in June, while at the same time launching a review of whether civil partnerships, only available to gay couples, should be open to heterosexual couples.
Mr Hopkins told MSPs that marriage is the "last big area in which the law still discriminates against LGBT [lesbian, gay, bisexual and transgender] people".
John Deighan, parliamentary officer of the Catholic Bishops' Conference of Scotland, said: "There is a fundamental philosophical clash of opinions here. There is a view that marriage is just a loving relationship between any two people who have committed themselves to each other.
"We don't change the whole of society because people might feel upset that their relationship isn't being given a particular badge by the state."