Paternity and Ultrasounds, some help?

Before my wife and I got married, We were still having open relationships. We slept together unprotected always and I always came in her. But, I have known for a while that I have a low sperm count, which is why she never got pregnant for the first 8 months we messed around unprotected. Well, right before we decided to be exclusive with eachother, she slept with her other "friend with benefits" twice and they used a condom both times. The next night her and I slept together without protection several times. We decide to be "together" and a few weeks later she tells me she feels sick. Takes a test, shes pregnant. The ultrasound at 8 weeks says she got pregnant the night she was with the other guy, even though the condoms never broke,. Could this baby still be mine since we had unprotected sex the night afterwards? How accurate are ultrasounds done at 8 weeks in determining conception? I still went on and married her regardless, and I want to be there regardless. I am just getting so excited as her due date gets closer and closer. I really want this baby to be my biological son, so I don't have to deal with the other guy trying to step in. any stories, input would be appreciated. thank you