So being touched by seung do was okay ofc because he enjoyed the pleasure and Seung do is someone familiar to him, he responded appropriately and backed off when he saw the bottom (so sorry I’m blanking on his name) get scared and push him. So he allowed him to continue because he showed restraint, a sign of respect for his autonomy. The more they did it the more they got used to it. Kissing, touching, petting it all becomes normal for him. The flip switched for him when 1. He saw him willing to help him and ready to protect him so his trust grew for him, he felt it was okay to get closer and 2. He saw how Seungdo was with his daughter and how he quickly accepted the uncle role. It’s like when someone is dating someone else with a kid and they say “I love your child because they’re part of you” obviously he didn’t express that but it’s the principal, him having kids changed nothing, his love never wavered, he wants to help him not for brownie points or to get in his pants but because he wants to, because he loves him.
So in my opinion that’s what it was. Seeing someone that you know is interested in you taking initiative, accepting you as you are baggage and all, going at your pace following your lead, being patient, and still wanting you, willing to protect you, and look at you no differently. I’d be sucking dick too LOL. But no fr he’s allowing himself to open up to him and respond to his feelings by way of being more forward with sex because he’s not good with vocalizing his thoughts at least the romantic ones. I don’t think they’ll go all the way until the secret is out though that’s the one wall he’ll probably hold firm on. Just to make sure he doesn’t put out for someone that will make him feel like a freak or disposable and ultimately leave him.

Ok, idk if I'm stupid and can't read between the lines, or I have to just wait for more chapters to see black hair's POV, but how did he go from scared and shy and refusing to talk to the one initiating physical things so casually