The buzz was about the sex, but the substance was all about the pain.

HBO’s “Tell Me You Love Me” offered lovemaking scenes so realistic (if not conventionally titillating) that they inevitably became the topic of previews and reviews — much to the bafflement of showrunner Cynthia Mort, who expected attention to fall elsewhere.

“No one believes that,” Mort says of her surprise. “Everyone thinks that ‘you must have known,’ but I didn’t really. I wasn’t prepared. There was such a tremendous focus on the sex: ‘Is it real?’ ”

It wasn’t, she makes clear — “They’re actors. They’re married, they have partners … you really believe they’re gonna have real sex?” And eventually, the lust settled to reveal the anguish at centerstage, featuring riddles like, as Mort says, “My husband won’t fuck me, but I love him.”

“Hopefully, it’s true enough and honest enough to be a reflection of what many married couples go through,” she adds. “It was never, ‘I want to get divorced.’ It was always this pain within the relationship.

“I wanted to examine people that stay in the marriage. … I was interested in showing that day-to-day stuff you go through in long-term relations, and how you come out and can you come out?”

Mort linked the series’ three principal couples with a common therapist and positioned their stories in a way that would allow her to explore “different stages of one relationship” — pre-engagement Jamie and Hugo, would-be parents Palek and Carolyn and full-on family folk David and Katie — engineering an unnerving game of “Can I spot myself” for auds.

Of course, that game was most unsettling for Mort’s friends.

“I would say, ‘You guys, I think I’m using you as some kind of inspiration,’ and they would say, ‘Don’t!'” Mort recalls with a laugh. “But (the characters) aren’t even composites; they are fully created fictional people.”

Best episode: Episode 10, the season finale. “I love the last episode,” Mort says, “because I just felt in every way I was able to bring these characters to who they were more than anything. I thought the acting was amazing, and the direction was incredible and it came together so beautifully.”

Underrated character: “I think maybe Jamie. I think she had a lot to do and brought unexpected depth to that journey.”

Great line: “I guess, yeah, I should be in the mood every time I clean out the gecko cage,” says Dave (Tim DeKay). “Everybody else is, it seems. I’ll tell you what turns me on, buying Cheerios is really hot. And then of course getting shoelaces or fantasizing about minivans — that’s sexy, too. … Then after all that, then I am ready to have sex, because all that is hot.”