It's great that Sunny is aroused by having his nipples exposed in such an erotic way.💕
Thinking about the tension in his pants is really good too.
Sunny, who has lived as a shut-in for a long time where he sleeps and masturbates often to pass the time, suddenly has to be outside and around other people a lot.
As such in a drawing like this, I think it is fun to imagine that he is horny often and has to deal with the stress of holding back his urges when he isn't used to doing so (and that Kel is too oblivious to notice and continues dragging him from place to place).