People and their emotions. Sheesh. This is a simple contractual issue.
Your boyfriend lent you the $2000 under the condition that you would pay it back. This is a contract, even if you didn't get it in writing. I assume you did not contract with him that he would remain faithful as a condition of paying back the loan, and that if he was unfaithful, you would not be required to pay back the loan.
Your boyfriend having cheating on you violated the agreement as to your relationship -- not the agreement as to the loan. The loan is an entirely separate issue.
It sucks that he played, but you still have to pay.
I'm not going to take the moral high ground when the facts are so clear. If you can't or won't pay him back, he can take you to small claims court. This was not a gift, it was a loan. And (depending where you are) it's likely that if he took you to court and won, you'd have to pay back the loan with interest and pay all his court costs -- immediately.
My best advice to you is to pay as much as you can and come to an agreement as to the rest -- if he feels badly about cheating, perhaps he'd be inclined toward being lenient as to the repayment of the loan. But it is still your obligation to pay the loan in full.
I had to kind of step outside myself to answer this objectively, as I was in a situation where an ex owed me unpaid rent and money for items of mine that he sold without my knowledge or permission. I was furious about this for months. But I went through the appropriate channels to get back the money, which was in the form of the security deposit... and lo and behold, it added up to within $20 of the approximate value of the items he stole and the back rent he owed. The law of karma - you get back what you give - worked as perfectly as always... though I could have used the interest on that money!
Anyone that incurs a debt is obligated to repay it. Period. Unfortunately, suing someone for cheating isn't really possible. Here's a bit of a variation on that... though do note that there's no legal advice in that link, and it would be moot because you weren't married.
Can you sue the other woman?
For revenge: living well is the best kind. Now you're free of an asshole who had sex with an acquaintance! You can find someone who treats you the way you deserve to be treated. And the $2000 you would have withheld would have been inadequate compensation for 5 years of your life, anyways.
Good luck to you as you move forward to making better decisions.