Two refreshing couples have an interesting take on the aftermaths of what they agreed to consider cheating.
Lucky Parasise and Love Bite
https://www.mangago.me/read-manga/love_bite_lamyo/
Just as you say , not sad nor tragic >> Love Bite and Lucky Paradise _ Wet Sand _ Love or Hate,
https://www.mangago.me/read-manga/neighbor_1/ (with two hypersexual guys that may become <ethical sluts> and polyamorous (or not) _ This one is a classic manga, highly recommend : A man falls in love with his slutty uke boss in the Yakuza world >> https://www.mangago.me/read-manga/saezuru_tori_wa_habatakanai/
I highly recommend < Love in Kitch > if you want a light hearted comedy +yaoi, it'really good ! It's cheaty ish without being cheating, you will see what i mean after reading the 3 first chapters.
Any well rated BL’s with cheating’ ish / someone not very committed to the relationship ?
Like say « the 10 years I loved you the most ». Doesn’t have to be sad nor tragic tho.