Choosing to have sex with only one other person. One way to prevent STDs is to have two people test negative and then stay together as partners. They agree not to have sex with a different person while with that partner. If this happens no one has to worry about getting a STD. Many people might only have sex with one partner at a time but if they have many partners, one after the other, they could get an STD if EACH partner isn't tested and treated.