i disagree, i think as the Mangaka implies that they were working for 3 days straight and didn't sleep either and the seme wasn't feeling well but accepted the invitation anyways because he was madly in love. so they probably died out of exhaustion and heart attack maybe
yeah lack of sleep is hard on your heart, so it's understandable that is was the cause of death--especially after multiple rounds of sex (rigorous activity)
The tails were so adorable and how they wrapped it around their lovers(≧∀≦)
Basically, it's the ukes fault because he invited for drinks after so much work. He should have just made it for the next day so they could at least rest a little
So I think it’s weird how they just died during sex... what I believe is that the uke loved the seme a lil more than the seme and couldn’t stand seeing all those girlfriends so he asked the seme if he wanted to come over for a drink. Then he got them the alcohol he put poison in it and while they had sex they died because of the poison in the alcohol and that’s the real reason he became a devil. I’m just sayin. (〜￣△￣)〜