Oprah Winfrey and Gayle King say they're a couple of straight shooters who'd admit if they were a gay couple -- a rare denial of a longtime rumor that's swirled around them.

The talk show hosts sat down for an interview with Melinda Gates on her "Moments That Make Us" series ... touching on rumors the two are actually in a long-term, covert relationship.

Oprah admits the pair faced questions about the nature of their relationship for years ... and Gayle jokes she used to beg Oprah to do a show about them not being THAT close, because the rumor was making it impossible for her to get dates.

Winfrey says she thinks people aren't used to seeing women with such a close "truth bond" ... so, they think the two must be gettin' it on behind closed doors.

Gayle says she always assumed every woman had a really close bond with another woman like she has with Oprah ... someone who cares as much -- if not more -- about their friend's success than their own.

Of course, everyone knows about the rumors regarding Gaylah/Kinfrey (whatever couple name ya like for the pair) ... in fact, Oprah shut down rumors about her sexuality while speaking with Andy Cohen a couple years ago.

Cohen asked Oprah if she'd ever "taken a dip in the lady pond" to which she unequivocally said no ... Cohen later revealed asking Oprah that question is one of his biggest regrets.

Sooo ... sorry to anyone who's still shipping the pair -- they say it just ain't happening.