Ultrasounds are just an estimate. All my babies have measured several days behind what my original due date was. I knew exact conception date so we didn't change dates. I just have small babies.
The dates are way too close for anyone to tell you will have to get a DNA test.
I agree with rdc4862. The dates are very close you cannot tell by that, only DNA tests after you're baby Is born Is going to give you an answer.
An ultrasound is not that precise of a way to measure when you conceived once you have passed about week 7 or so of pregnancy. By week 12 it can be a week off for purposes of determining a conception date. A later ultrasound should not be used to negate what the earlier ultrasound suggested. Unfortunately in your case, if I remember correctly, sperm from guy 1 could have still been in your system (if not too lively) by the time you had sex with your husband, and your egg can last up to 36 hours before being fertilized. Meaning, everything that points to one guy is countered by something else.
The "Shettles method" would suggest (NOT guarantee) that if the baby is a girl, it is more likely to be from the sex on the 8th, and that a baby from the sex on the 13th would more likely to be a boy, given when your possible ovulation was. But again those would only be suggestions and not a guarantee by any means. People try using the Shettles approach to time sex in order to have one particular gender of child all the time and it doesn't work all that well.
You will need a DNA test to confirm paternity. I'm sorry there is not a clearer answer for you. You can get such a test (blood draw from the mother's arm, swab from the dad, I believe) before the baby is born, but it is about a tenth as expensive if you do it after the baby comes.