It's an unfortunate myth that using condoms will keep you safe.
I have always used condoms in every relationship until everyone passed their tests. And then six months ago, I got an abnormal pap smear.
Several hours of research later, I learned that HPV (Human Papilloma Virus) can be passed on when using condoms AND it's not covered by standard STD testing. In most places, you can't even get access to an HPV test if you want to test for it specifically. Until you get an abnormal pap smear, HPV is completely asymptomatic (unlike Herpes, which can be transmitted with condoms, but usually gives you a visual indicator you're infected).
In fact, over 90% of sexually active adults have had HPV at some point in their lives, and up to 70% of adults have it at any given time. In most cases, the body treats it like any other viral infection like a cold or flu, and you're all better in no time. But in some cases, it sticks around and can cause cervical cancer.
Don't get me wrong, I'm obviously not saying that using condoms is pointless, they still do a great job at preventing HIV, gonorrhoea, syphilis, and babies... but don't think that using condoms will completely protect either yourself or your partners. AND don't assume that because your STD test comes back clean, that you don't have any sexually transmittable infections.
So unless you and your spouse both subscribe to no-sex-before-monogamous-marriage, then you're always at risk of transmitting something to anyone you have sex with, or having them transmit it to you.
Bottom-line: Any time you're having sex with other people, you're risking bringing something home to your partner.