A lot of trust and just plain luck. I never used protection and my wife was always fucked bareback by me and every other guy that had her. That being said, if you do enough fucking around, especially with people you don't know well (and even some that you do), you WILL catch something sooner or later. Being careful in you choices is paramount to reducing this. You can never know your sex partners too well when it comes to not catching anything.
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