I have been looking over everything to be sure of the details. One question that never got asked is if you had sex between March 23 and April 1. You began by saying you slept with one guy April 1 and your boyfriend on April 4 -- had you also been with your boyfriend before April 1?
OK, so, you said the first pregnancy center told you on May 21 that you were 8 weeks 3 days GA, and that you your due date was December 24. There is a discrepancy because on May 21 if they were saying you were due December 24, they should have said you were around 7 weeks 6 days. (You're positive this appointment was May 21, right?) Medical places don't usually give a GA date (the "weeks pregnant" figure) that doesn't match the date you are due, unless the GA count came from your report of when your last period was, and then their ultrasound showed something different and they corrected it and gave you the due date based on the ultrasound data. In any case, once you have an ultrasound, the date to believe is the one from the ultrasound, not a date counted forward from a period.
Regarding the December 20 estimated due date that your doctor gave you after an ultrasound, how much later in the pregnancy was that? It doesn't necessarily conflict with the first due date if it was past your 12th week, because by then there is a margin for error of +/- 7 days when trying to determine a conception date by ultrasound. (Some babies grow faster than others as a pregnancy progresses.)
Unfortunately, not a lot of this points to the sex on April 4 as producing the pregnancy, though the margins are pretty slim. If you were sleeping with your boyfriend throughout, (in other words, within the week after March 23), then you're back to it being an even chance for either guy. But if you didn't begin with your boyfriend until April 4, the first guy's chances are greater.
It's too bad you didn't have your ultrasound in your 6th or 7th week; by week 12, doctors give a week's margin for error when trying to determine conception by ultrasound measurements. (By week 40 the margin for error is plus or minus three weeks!) But the info you do have does suggest that if your only options for having gotten pregnant were the sex on April 1 and April 4-5, the sex on April 1 is more likely.
Hi, if the sex with both guys was unprotected, and if you assume that you ovulated around then, it could be either guy. To try to get a handle on whether you had ovulated around then: what day did your last period come? Are your cycles regular? About how many days apart are your periods?