I'm not understanding why you feel betrayed. You were not together and it was 5 years ago. Were you two having protected sex? If so, he was doing what he has always done, if not, he spent some time being careless. At any rate 5 years ago is too long ago to be tripping now. I think you need to ask yourself why this is bothering you now. Do you think he is cheating now? Is the relationship good now? Are you questioning why you are back together?
Just my opinion, but his unprotected sex 5 years ago is not the question, but rather why you are feeling this way.