I asked my bf if he wanted to sleep with other girls/have a 3some, and he said no. Is he lying?

You put some thought into this huh? You are way to practical and realistic... Trying to calculate all the possiblilties and such of what he might "really think" because of what he said before. Calm down. If you believe him then you trust him. To me it sounds like your doubting him to much or you just dont have confidence in yourself. And yes guys can see an attractive girl but still have no sexual intentions. Its called self control. Your boyfriend got it as soon as he became honest and loyal to you. -.-