It's a good suggestion-- in such a position, with Byleth atop him, he would be able to entirely dictate the pace of their encounter. That would mitigate any potential risk of Dedue hurting him unintentionally, either from his own inexperience or his own unwieldy anatomy.

It seems that Byleth is full of good suggestions, really, and the next one that he suggests, that Dedue test him with his fingers, is intriguing on many levels. On a purely practical one, it would help to ensure that Byleth is adequately prepared for their coming activities; on a less purely practical one, Byleth is a beautiful man and Dedue would very much like to please him. As though to add credence to his argument, the gentle tease of the Professor's fingers is pleasant, makes him feel warm.

"As you like," he replies, and settles on the bed with Byleth. This will be fine; he simply has to trust in the Professor, as he has done many times before. Trust him and undress him, gently tugging his boxers down over his pale hips.

"You are lovely," he says, every word of it honest, and leans in to kiss him. His hand settles warm and heavy on Byleth's thigh, large enough that most of its width fits within his grasp.