That's what I assumed as well, that it was 2% per incident, but I'm having a little trouble parsing those differently:
How is 2% per incident different than 2% per year? I'd interpret both of those statements as 'on average, given perfect use, a condom will be ineffective at preventing pregnancy in one use out of fifty'.
Well if you have sex once per year, it is not different.
My condolences.
P/S/A: There are single sentences which can create life-changing amounts of difference.