Well, it means get a DNA test, but it also suggests the guy on the 2nd was a week too early to get you pregnant. That's not a wide enough margin to be totally sure, but it is wide enough (given your very early ultrasound) to feel pretty confident it was the second guy's baby.
This is what I get from the conception calculator I use, for an estimated due date of September 1. If you got your measurements from an ultrasound when you were only in your 7th week, it should be pretty reliable.
Probable date of ovulation: December 9
Possible dates of conception: December 5 to 13
Due date: September 1 (40 weeks)
Yes they estimated September 1 @ 7 weeks
Do you have any data from an early ultrasound, like in your 7th or 8th week? Because counting forward from your last period is not going to be precise enough to pinpoint the difference between December 2 and December 4. If you had an early ultrasound, what was the estimated due date that they gave you at that time, and how many weeks pregnant were you?