It's hard to know what SW was thinking. Maybe he thought the condoms were just for birth control and that you didn't need them anymore? Even if you told him it's your agreement that you wouldn't have unprotected sex, for some people "protected" only means "from a pregnancy".
While it was wrong of him in the heat of the moment to slip it in, he did ask if it was okay pretty soon, just in case. He wasn't clear enough, you weren't paying attention enough, mistakes were made. I think you didn't break your agreement any more than someone who was raped broke an exclusivity agreement (not saying you were raped, just that you didn't choose to have unprotected sex, it happened to you).
You had all reasons to believe he would slip on a condom as usual. Now you'll know to be more careful about that, but trusting him was reasonable.
Now, about the pregnancy option. Did you get a morning after pill? If you're not pregnant yet it can prevent it. They can have some effect for up to 3 days after the sex I believe (as it's the latest it can take to get pregnant, by then the sperms die). It's less effective the more time passes (because it doesn't work if you're already pregnant) but it can help if you still have time to do it.
Otherwise, I don't know what to say. I can totally understand your position (pro-choice, but not feeling able to get an abortion) but I think you should really consider the options if you do learn that you are pregnant. Would the child have a good life being adopted? Would you? Would the pregnancy have negative effects on your life? Abortion can be hard emotionally, but giving up a baby can be harder. Etc.
It's of course YOUR choice, but I think it might be worth thinking about it deeply, since the consequences will be forever.
Have a talk with SW if it hasn't happened yet, make sure you're clear now. Reiterate that you want condoms to always be used, and make sure to check it happens from now on. The only thing you can do now is make sure this doesn't happen again.
If you are pregnant, consider asking him if he would want to raise the baby with his girlfriend (or on his own), as that would be simpler than adoption and you could have a role in the child's life without being a primary parent.