Hi, where did you get the computations of April 13, 5 weeks 5 days, and May 4, 9 weeks 5 days? Did you have ultrasounds?
Sex on cycle day 6 is not in the traditional fertile window and has a low probability for pregnancy. Sex on cycle day 9 is close enough to ovulation CD 14 that pregnancy is possible. Sperm can last inside for 5 days. Couple that with the info that the first was protected and the second not, there is little doubt the second guy is the lucky one. The ultrasound dating confirms that ovulation was on schedule about the 18th or 19th.
I see why you ask. It means I think the second one is the dad. I can’t tell from your post which one you prefer.
I keep thinking about the fact that you said "I had my first ultrasound appointment April 13 and they said I measured around 5 weeks and 5 days but it was very early in the pregnancy so the yolk sac wasn’t formed yet." By 5 weeks 5 days, there would have been a yolk sac, and also a fetal pole and heartbeat. This suggests you were not as far along as 5 weeks 5 days. (Even though, on April 13, you would have been 5 weeks 5 days from the first day of your last period, and until an ultrasound that would be how a doc would compute your weeks GA.) This is why I asked above which information you were given by a medical person looking at an ultrasound, and if you computed any of it yourself.
If the 9 weeks 5 days GA figure was given to you on May 14 by a doctor looking at your ultrasound, that suggests conception around the 21 of March, plus or minus 2 or 3 days. This would be consistent with your earlier ultrasound showing a baby not quite as far along as 5w5d on April 13. The 21st of March would rule out the guy on March 10, but it also would tend to rule out the guy on the 13th, unless that margin for error comes into play. That is why I asked above if March 13 was the last time you had sex before learning you were pregnant.
In short, if everything is as you say, with the April 13 ultrasound seeing no fetal pole nor heartbeat, the May 14 ultrasound indicating you were 9w5d gestational age, and with all of this information given to you by a doctor or tech reading the ultrasound, and none of it from you figuring out the weeks or due date yourself, then someone with whom you had sex in the week after the 13th (if there was someone) is more likely to be the dad than the guy on the 13th.
It sounds like you didn't get pregnant by the guy on the 10th, in any case. Given that you had no sex after March 13, your early ultrasound results are surprising -- the lack of a fetal pole and heartbeat would fit the dates better if the sex with the last guy had been on March 15th, or 18th, or even as late as the 21st. But they definitely don't suggest the baby came from sex on March 10. Since the sex on the 13th was the last sex you had, (and of course since it was unprotected and the other sex was protected), in your shoes I'd go through the pregnancy feeling pretty sure the guy from the 13th is the dad.
If you're not married, do a DNA test when the baby comes so the legal father will be on the baby's medical record. It will also have the benefit that you'll never have to wonder about this again.