The video embedded below is of Ian Ayres at a TEDxYale event this year and it is worth 10 minutes of your time. His main point is that we can greatly reduce the transmission of STDs by always using a condom at least the first three times you have sex with an individual partner. Note that he is not suggesting that you should have unprotected sex after your third sexual encounter with that partner. After your first three sexual encounters with an individual you should make educated decisions about how you continue your sexual activity with that partner. By doing this we will decrease the number of times we have sex without a condom and, with widespread adoption of the Rule of 3, the spread of STDs can be meaningfully slowed.
As I previously mentioned, this video is easily worth 10 minutes of your time. I also encourage you to share this video with others (it is hosted on Youtube)