Safe sex
Safe Sex: Sticking your penis in and out of a heavy metal box with a combination lock on the door would be safe sex: Safe sex means staying away from diseases and infections during sex.
Not having sex is safe, but it is not sex.
No form of sex is 100% safe. Reasonably safe sex is knowing your partner's sexual history is clean and taking precautions to limit the likelihood of spreading a disease or getting the woman pregnant.
Condoms are used during reasonably safe sex. A thin piece of latex is not 100% safe. Failure rates are reportedly around 10-15% and that is for the correct use of them. (ref: 4parents.gov) During the heat of the moment, failure rates may be higher due to misuse of the condom.
There is no such thing as SAFE SEX, but reasonable precautions can make sex safer.