
Makio is one of the MCs of "More than Words" by the same author. It was adapted fully into a jp drama in around 7-8 eps back in 2022 which also adapted the 1st vol of this manga in the remaining episodes of the 10-episode drama.
The kid is not Makio's but he feels responsible for it for reasons explained there.
Major Spoilers for More than Words below.
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The dark haired guy, Makio, had a bf and a girl best friend bck when he was in hs. His bf was already in college at the time when bf's dad found out about his son's relationship with Makio. Dad was not happy since he wants everything to be heteronormative. The best friend, upon seeing what her friends are going through, stupidly suggested to bear a child for Makio and the bf. Makio was against it ofc, but the girl is so assertive about it and also has abandonment issues that ig could explain her want to insert herself in her only friend's life. The Bf was also no better. He's even enthusiastic about it, disregarding his lover's opinion.
Some months later, they found out the kid is the bf's. Makio, witnessing the growing intimacy between his bf and bff and couldn't bear what is happening any longer, left. In Makio's own words, "he ran away".

It addresses everything. Issues on sexuality, even the preference on who wants to take the lead in sex. It's one of those mature romance that shows the sweetness and joy of being in love with all the good things, and the trivial annoyances. Without being patronizing on how to make a relationship work, or the hardship of being in a same-sex relationship in a conservative society.
Ako naman lord ganito please.