I think the obvious disadvantage is that you might have gotten an STD/STI (not sure what the exact difference is aside from their name but they are all that bad stuff that can be sexually transmitted) and not even know it. Not knowing if you have an STD/STI could lead you to potentially giving it to future partners and cause you symptoms from that STD/STI which can be anything from mild irritation to detrimental things that could affect your daily life, if you'll ever have children in the future, etc.
Another disadvantage is that you or a partner might get an unwanted pregnant and be unaware of it until it's too late to get an abortion or whatever (that is if you are referring to tested as in a pregnancy test). That can obviously cause more issues especially if you and your partner are not ready to be parents.