This manga needs the following tags: NTR, Weak Male Character
In short its a manga version of the creators(cuck) OG story which you can find on pixiv. Which means it has less sex scenes, (either completly removed or strongly shorten) then the original which makes no sense to read it. The only thing what MIGHT be different is how the story CAN differ from the OG story, right now I can say its the 100% same. Which brings me to the last point, if you dont like NTR (because this happens through the whole story), dont like a beta cuck MC simping for a slut, or dont like to see other + the main Love interest having sex ( she fucks like dozens of guys at the same time), you better drop it since "A 100% pure romcom that starts from NTR" is only bait. There is no romance, there is no comedy, there is nothing good to say about either MC or FMC since its more or less a NTR fetish manga/hentai which tries to downplay its disgusting premise.
In reality is all about MC simping and aiming for the slut -> the girl in the cover, while she has sex with her BF all the time. At about 75% of the story MC somewhat wins her ower, BUT she still fucks her BF + other dudes because its written by a author who has this fetish if you check his Pixiv and Fanbox.