using sex as a medium to portray the progression of the relationship between characters definitely works as a "plot" lol. interpersonal relationship dramas are based on this. I think we shouldn't be so dismissive of human sexuality as a narrative choice considering how important sex has been for humans for as long as we've existed on this earth.
there are definitely authors who aren't good at showing the chemistry between 2 characters thru just smut, but they just honestly need to step their game up haha its totally possible
ofc there are totally books/manga/manwha that I've read just for the smut and skimmed over everything that wasn't smut bc the main plot just wasn't interesting (bad thinking diary, tada de wa dakaremasen) but that's just a preference, personally speaking

if the book has frequent sex scene, that goes on 3-4 chapters
would you get boring or not?
personally i like more plots and less frequent sex scene just make it more…romantic
i just want to know how is it for others