This isn't something he ever thought to ask for from Guilliman. Although he likes to think he's more progressive than a lot of men his age, he's not immune to certain arbitrary rules in relationships, either. When he submits to someone, he services them in this way but doesn't expect it in turn. When someone submits to him, it's the opposite.

But Guilliman - wonderful, perfect Guilliman - is taking care of him like this. Enthusiastically so.

"Oh," he moans, back arched, cock easily getting hard as his lover's tongue works it over, "Yes, please, keep going."

To know that he's the only one to receive this privilege is a delight in itself. Whatever relations Guilliman had in that other world don't concern him, he was hardly monogamous there himself, but he's elated to know that this belongs to him and him alone.