It occurs to me that the "we both forgot to use condoms" thing that comes up often enough is a bit like suggesting that a person forgot to wear pants.
For a whole day.
And didn't notice.
They got home, and after hours of being out in the cold or the sun, at work, getting lunch, taking the bus back and forth, only when they saw their pants laying on the bed did they go, "Whoah! I haven't had pants on ALL DAY! I had NO IDEA WHATSOEVER!"