That all said, you are right it is less likely to be the blackout-sex guy because your test said positive only 3 days after the sex. Are you certain of your dates? In other words, you are SURE the sex was on June 15 and you are SURE you took the pregnancy tests on June 18?
It's a close call, for sure. When did you get the due date of March 5, and how did you get it? Was it, I hope, from an early ultrasound? If so, how early in the pregnancy was it?