Sechel, what dovid did was not an aveirah, but even so, he did teahuva for what it looked like. Also, this wasn’t a slip up on the baaleo lashon hora – this was something they were actively engaging in consistently.
Dovid’s rebuke has nothing to do with the practical, down to earth difference between a man who violates aishes ish and a person who makes an offensive joke in public. He was driving home the severity of what they were doing – because it’s very serious. But the path of teshuva for one who does such “frum” sins is much easier and quicker than a person who is steeped in tumah, such as arayos or shfichas damim. On a certain level, lashon hora/etc.. are worse – but only in some ways. That’s what the baalei musar write on such statements of chazal, that they’re bringing out a certain bechina, nekudah that needs to be stressed, but are not speaking about the practical differences. For example, there’s an element of avodah zara in every sin, but not every sin is literal, pashut avodah zara. There’s an element of AZ specifically in anger, eating chometz on pesach, and others, but they’re not literally AZ.
Yes chazal say that a person who lends on ribis has no techias hamaysim; what’s your point? That we should think there’s no logic in Torah? Of course there is! It’s on us to understand why a person who does that forfeits their eternity – do you think Hashem takes away olam haba, the purpose for which He made a person, easily and in a way that we’re not privy to? How are we supposed to avoid losing our olam haba if we’re not given the tools necessary to do so?
Hashem wants us to understand it as much as we possibly can. It’s not catholic dogma which is just recited by robots. At a certain point, we say we can’t understand some details, and we just say it’s a gezeras hakasuv, etc…were not talking about maskilim here who need to understand something rationally to acccept it. I’m saying there most definitely is a logic in Torah, just at a certain point it’s beyond our comprehension.