A March 30 due date suggests conception on or around July 7 (depending on how early you got the due date). If it was the due date they gave you in your 7th or 8th week from an ultrasound, it rules out all possibility of the baby coming from sex in early June. Similarly, an April 5 due date gives you an estimated conception date around July 14. Those plus the fact that you had a period in June after the sex should more than just suggest to your husband that the sex in June had nothing to do with the pregnancy, they should spell it out.
At least we can work out the issue of whether you got pregnant in June if we do some computations:
What due date did they give you back in August when the ultrasound said 7 weeks 5 days GA? And do you know what day in August that was?
I only slept with my husband after that incident & I meant 7 w 3 days