WATCH: Rick Santorum Regrets 'Man on Dog' Remark — Sort Of

Rick Santorum has now expressed regret for his invocation of "man on dog" sex in a discussion of gay sex -- but he stands by the substance of the remark, if not the exact wording.

The subject came up when Santorum appeared Wednesday night for a one-on-one, in-studio interview with Rachel Maddow on her MSNBC show. Few Republicans have been willing to go on Maddow's program, but Santorum said he agreed to the interview because "You stick to the issues, and I respect that."

One issue that was bound to come up with the former U.S. senator and current GOP presidential hopeful was his staunch opposition to LGBT rights, especially as expressed in an infamous 2003 interview when the U.S. Supreme Court was considering the constitutionality of antisodomy laws in the Lawrence v. Texas case. He told an Associated Press reporter that if the court would rule that people have a right to any type of consensual sex in the privacy of their homes, it could find a right to polygamy, incest, or any number of other things, such as "man on child, man on dog, or whatever the case may be."

"Why did you say the word 'dog'?" Maddow challenged Santorum Wednesday night. He said he was quoting the majority opinion in a previous sodomy case before the Supreme Court, 1986's Bowers v. Hardwick (which, by the way, does not contain the word), then said, "I wish I'd never said that -- it was a flippant comment made to a reporter who was not being particularly professional. ... But the substance of what I said, which is what I've referred to, I stand by that." That is the slippery slope he has often referred to, and he recently said the Supreme Court's June marriage equality decision proved him right.

Santorum also told Maddow that he believes the Supreme Court doesn't have the final say on constitutional matters -- on which she likewise challenged him -- and he speculated that if Congress were to pass a law revoking marriage equality, it might be upheld by a future high court with a different set of justices. He said the pro-marriage equality decision authored by Justice Anthony Kennedy had no basis in the U.S. Constitution and created a right "out of whole cloth."

Another question Maddow put to Santorum was whether he believes sexual orientation is inborn or chosen. He said he doesn't really know but added that he knows people "who identified themselves as gay and lesbian and no longer are." When she pressed him further, asking if he chose to be heterosexual, he said, "I don't spend a lot of time thinking about these things." She came back by saying, "You're really very interested in gay rights, though. You talk about gay rights all the time. That's the whole reason you're a nationally famous figure." That led into the discussion of the "man on dog" comment.

Watch the segment below.