My partner and I have never had unprotected sex without the explicit intention of wanting to be pregnant, but the desire to do so has definitely been there for both of us before.
But while there is obviously good reason to avoid unprotected sex if you are with multiple partners, I think it is much less of a big deal if you are with the same partner and are willing to deal with the possibility of pregnancy.
...and just because I was curious I looked up the average chances of pregnancy with unprotected sex during a woman's cycle...
Here's what Princeton researchers say about the chances of pregnancy with unprotected (or failed birth control) sex:
Emergency Contraception: Risk of Pregnancy
"Researchers have calculated the average risk at different times in a woman’s monthly cycle, but it’s important to remember that your risk could be higher or lower. The risk is highest on the days right before you ovulate (when your ovaries release the egg), reaching a maximum of nearly 30%. You can’t know for sure when you will ovulate, so researchers have calculated the average risk of pregnancy based on a woman’s monthly cycle (since you can figure out when you started your last period).
During the first two days of a woman’s cycle (day 1 being your first day of bleeding), the average risk of pregnancy is virtually zero. After the first two days, the risk starts to rise steadily, reaching 9% on or about day 13. Then it begins to decline slowly until it levels off at about 1% on day 25. It stays at about 1% for the rest of your cycle. (The average monthly cycle lasts 29 days, but it is perfectly normal to have a cycle that lasts as little as 20 days or less or as long as 40 days or more.)"