Pleasehelp

I'm looking for this manga I read a while back. A high school guy meets an older guy. I'm pretty sure the older guy did something to injure the younger guy because the older guy forcibly plants himself at younger guys house to take care of him. I think the younger guy doesn't live with his parents. The older guy starts to fall in love with the younger guy; so, he masturbates in front of the younger guy and says something like, "this is how dirty adults are". Fast forward, they enter a relationship and I think older guy wants to wait till younger guy gets older before they pursue a sexual relationship. Eventually, older guy and younger guy try to have sex but younger guy is too afraid because it hurts. So, they stick to thigh gap sex. But younger guy finds out that older guy is having sex on the side because their simple sex isn't enough. All I remember is that younger guy sticks a sharpie marker in his butt and it gets stuck.