Sexually transmitted diseases come in all kinds of forms, from the relatively harmless to the life-threatening. Some, like chlamydia, are easy to treat if you find out you have them early on. Others, like genital warts, are ugly and annoying, but not generally very dangerous.
Yet others, like HIV, can kill you. And for all of them this holds true knowing that you have them is important. STD testing isn't just for folks who are not in a stable relationships, or for those who have had unsafe sex. At the risk of opening a taboo, I'll say it, because it's necessary: Not only "sluts" (male or female) get sexually transmitted diseases.
They can happen inside marriage, and to those who practice safe sex. Even if you trust your partner, and are very sure that he or she is not unfaithful, there is still something to be said for annual STD tests. Sometimes, your partner has an STD from before he met you that he didn't know about because STDs often have no symptoms. And sometimes, your partner does sleep around. Sometimes, condoms go wrong. With genital warts, they don't even offer full protection.
If you have an STD, the right way to deal with it is not to figuratively stick your head in the sand and pretend it doesn't exist. If you are worried about having an STD, not getting tested won't make it go away. On the other hand, diseases like chlamydia and gonorrhea, while they can cause a lot of damage (including infertility), can be easily treated with antibiotics in the early stages.
When you get tested for sexually transmitted diseases every single year, regardless of your relationship status or whether you practice "safe sex" (there is no such thing, remember!), you know that you are free of diseases or can get treatment early on if you do happen to have one.