I don't know your significant other, so I'm going to try to give you the things you need to think about.
Sex is only one way that sexually transmitted diseases can be spread. We can catch them through sharing needles or, very very rarely, through blood transfusions.
Statistically, you and your partner are almost certainly both totally disease free. However, there is a difference between someone who says he or she hasn't had sex and someone who hasn't had sex.
What I think we all say is that there is no such thing as absolutely safe sex. Basically, in your situation you're trusting your partner's word about his or her history. The odds of having one of the deadly diseases is literally one in millions for people who have never been sexually active and who don't use IV drugs. You have to make the decision if you're willing to trust your partner's word with your life.
You're doing the right thing to educate yourself about the risks and ways to prevent problems. Chances are you're both fine but you always need to be aware of the risks. I don't wear a seatbelt because I think I'll be in a wreck. I wear it just in case.