Any recommendation for manga/manhwa that have fun side couple stories, for example that I ...

Don't know if you will like it, but enjoy the side couple in "Love so Pure"

Multi couple BL

Yaoi mangas:
https://www.mangago.me/read-manga/tadaima_okaeri/ (not exactly smut but it has implied scenes)
https://www.mangago.me/read-manga/encirclement_love/
https://www.mangago.me/read-manga/ichika_bachika/

Yaoi webtoons/manhwas/huas:
https://www.mangago.me/read-manga/highest_order/
https://www.mangago.me/read-manga/bj_alex/

Shounen ai manga:
https://www.mangago.me/read-manga/sasaki_to_miyano/

Shounen ai webtoons/manhwas/huas:
https://www.mangago.me/read-manga/blunder_turned_wonder/
https://www.mangago.me/read-manga/19_days/
https://www.mangago.me/read-manga/my_four_masters/
https://www.mangago.me/read-manga/merry_marbling/
https://www.mangago.me/read-manga/snail_pond_robber/

Here’s a bunch dear! Hope you enjoy! (≧∀≦)