Kent doesn't even have a complete understanding of Primordials, so there was just no way he could understand Eternals.
This was why, when Fiannarel said that absorbing her Yin Essence was impossible, he believed her. The last thing he wanted was to do something he would regret.
While sleeping with her isn't something he would regret, he knows the moment he absorbs her yin essence - something he had no control over - the danger will follow, and he is not ready for it.
It was similar to when he first slept with Vexthra.
Back then, the yin essence he absorbed was stored for him. However, things are different now, and Kent knew he had to be extra careful.
Perhaps if she were a Primordial, even if she were a Primordial King, he would have managed somehow.
"You do not have to worry, big sister. It wouldn't be long before I become a god. Then I can have your virginity and have all the sex in the world," said Kent, who was already looking forward to that day.
