Byleth may, in fact, be attempting to do what many people have tried and failed in the past-- to kill Dedue stone dead. His current methods may be considerably more effective than those before him, as well, because though Dedue may be impervious to most physical forms of attack, he is not impervious to expiring due to excessive lewdness in his vicinity. Byleth touches him with such reverence and gentleness despite the incredible horniness of his actions, and it's a dichotomy that threatens to override his brain entirely. He could, perhaps, be forgiven of the way that he trembles and has to focus so hard on simply maintaining control over himself, since he's so rarely been on the receiving end of such things. Once before, and not nearly so... lasciviously done.

For lack of a better way to describe it, Byleth sucks his cock. Dedue's hands clench in the bed sheets so tightly that he threatens to tear the cloth.

Byleth asks him to lay with him, and though he still has some misgivings about all of this-- he still seems so large in comparison to the Professor-- he should trust in his judgment. And it's very difficult to refuse a man who's got his hand on your cock.

"I. You will tell me if I am too rough with you?" He cups Byleth's face gently with one hand, keeping enough control of himself not to shake.