No penetration equals death?!

The eldest of the six brothers, Fukakusa Riyuu is an omega who supports his family after their parents passed away accidentally.
“Find your destined pair and you’ll see a drastic change in your life.” says a mystic website he browses. And the following day, just as the site predicts, hot shot alpha Harumi Jou shows up on his doorstep!
The two get instinctively attracted and start to mate in order to be a pair, however, no matter how hard he tries, Jou can’t penetrate Riyuu!! And then they notice the disclaimer in the mystic website… “In case you fail to become a pair, you both could die.” !!!