You have to assume that even with pulling out, your boyfriend's sperm might have gotten into your body when you had sex. You don't say whether the sex with the guy at the friend's house was protected, but it sounds like not. Even if he wore a condom, protection is not guaranteed. That means you could have had sperm from both guys in your body at the same time, on May 2, which is right when you would have ovulated according to your ultrasound. In short, it's too close to call.
A guy can have sperm in his urethra from a prior ejaculation, and when he gets an erection with you, the sperm can ride into your body in his pre-ejaculatory fluid. As of May 1, you could have had lots of living sperm in your body from your boyfriend (it can live in your reproductive tract for several days). Then you had sex with the second guy. So there would have been living sperm in your body from both guys.
Unless you have the funds to do a prenatal DNA test, unfortunately you won't be able to know who the dad is for sure until you do a DNA test once the baby comes (it costs about a tenth as much as doing one before the baby is born). I'm sorry, but there is no other way to know at this point.
Agreed, you could have had sperm in you from either or both guys when the egg came. A paternity test is the only way to know the father. If you fear revealing your infidelity to your boyfriend you could ask the casual guy to help out.