Unstoppable Siblings. One of the side couples is lesbian.
I don’t know if this is what you’re looking for but, ML is in a straight relationship rn and there is a whole story about him and his male ex as well.
http://www.mangago.me/read-manga/an_innocent_sin/?t8CA
Also there is are gay characters on the side, but I don’t know if they turned out to be a couple.
http://www.mangago.me/read-manga/the_missing_o/
Its smut, lesbian side couple
Sotries where the main couple is straight and the secondary couple is gay. Or the other way around.
And when i say secondary i mean like their own plot line like with Jazz for two with that Doyoon guy and the other dude. That big.