WASHINGTON — The campaign trail can bring fraught encounters for a presidential candidate. At a televised forum on L.G.B.T.Q. issues on Thursday, Senator Elizabeth Warren was asked to consider a hypothetical one.

Suppose a supporter approached her and said: “Senator, I’m old-fashioned and my faith teaches me that marriage is between one man and one woman.”

How would she respond?

“Well, I’m going to assume it’s a guy who said that,” Ms. Warren replied. “And I’m going to say, then just marry one woman. I’m cool with that.”