A forum participant asked her: “Let’s say you’re on the campaign trail and a supporter approaches you and says ‘Senator, I’m old fashioned and my faith teaches me that marriage is between one man and one woman.
“What is your response?”
“Well, I’m going to assume it’s a guy who said that,” the politician said.
“And I’m going to say…then just marry one woman. I’m cool with that!”
After a brief pause for raucous applause, she added: “Assuming you can find one.”
Prior to the event, the senator released a plan to further secure LGBT+ rights and declared her support for the Equality Act.
The bill, if passed by Congress, would explicitly add LGBT+ protections to federal anti-discrimination law. It is backed by most Democrats.
The act was passed by the House but is yet to be considered by the Republican-controlled Senate.
Ms Warren also promised to elevate LGBT+ concerns by appointing a special envoy at the State Department.
Kamala Harris and Pete Buttigieg, her competitors, also declared their support for the Equality Act.
All three candidates also said they would make pre-exposure prophylaxis, a drug which helps lower the risk of contracting HIV, accessible and affordable for all.
The televised CNN town hall was the second major presidential forum dedicated to LGBT+ topics held during the Democratic campaign.
An event in Iowa last month drew 10 of the 19 Democrats hoping to take on Donald Trump in the 2020 election.
Additional reporting by agencies