So Darien's heart condition isn't fatal .. well, it's extremely RARE it's fatal.
"In Wolff-Parkinson-White (WPW) syndrome, an extra electrical pathway between your heart's upper chambers (atria) and lower chambers (ventricles) causes a rapid heartbeat (tachycardia)."

Seb could also discover it mid-beginning sexy time and be like "I wonder what would happen?" xD Actually! SURPRISE ME! Go in the direction you think would be fitting for Seb's personality, or what would be fun. :D