Thank you for the opportunity to help you with your question!
1. The concept of "safe sex" arose from the HIV, AIDS and other viruses transmitted through sexual intercourse. Female and male condoms prevent penetration of sperm into vagina/uterus.
2. Sexual behavior affects transmission of STDs because many people have sex when they are "going with the moment". Many young adults, teens don't consider the consequences of sex other than possibly becoming pregnant and using birth control. STDs such as herpes, HIV, AIDS and other diseases must be taken seriously if partners have "loose' sexual behavior (multiple sexual partners). Condoms can prevent transmission and should be used at all times whenever you are dealing with a partner with an unknown sexual past. It's much safer. For people with multiple sexual partners, it is also important to discuss possible transmission from other partners and get tested every 6 months or so for STDs if not wearing condoms on a regular basis.
Please let me know if you need any clarification. I'm always happy to answer your questions.