I agree that consent is a very serious topic and in real world stories it should always be dealt with as such. However, in a world where people go through stages where their quite literal “animal instincts” come out, I feel a bit of grace should be given to the characters. Especially if the recipient himself understands this fact and bears no ill will.
I think that drawing proved to Seyeong that Heejae was actually serious about him and that’s why he got all blushy ε=ε=(ノ≧∇≦)ノ