No Guarantee
Condoms cannot guarantee that you will not be exposed or get infected for several important reasons:
Condoms do not cover the entire genital area
Contact with infected skin alone can lead to the spread of disease especially with viruses and lice. Any genital contact, therefore, puts you at risk. But remember, not wearing a condom leaves you wide open for all infections or pregnancy.
Condoms break
If you do not place the condom on properly or use a defective one it is more likely to break and you or your partner could be exposed to an infection.
Many people have foreplay naked. Release of semen (pre-cum), contact with infected skin or vaginal discharge before sex can lead to pregnancy or disease.
Many people have oral and anal sex without condoms
You can also get STDs in your anus, mouth, throat and tonsils. Since the tissue in the rectum is very delicate it is more likely to break and the germs can spread into the body through the tears. You may also have tears in your mouth (from brushing/flossing your teeth) where germs can enter. Avoid brushing/flossing at least one hour before sexual contact.
Some people get caught up and think one time without it wont hurt WRONG!!! You can get HIV, any other STD or pregnant your first time.
He Can "Pull Out" and Climax Outside of Me
WRONG, AGAIN!! The same fluids that can infect you on the inside can infect you on the outside. Many people get warts, Herpes or Syphilis cause someone "came" on their genital area, back or stomach.