If you're going to have sex with other people, yes, you can REDUCE the risk of STDs, but you cannot completely eliminate that risk.
You can control what you do, but you can't control what others do, especially in the relatively private realm of sexuality.
That's why, regardless of what level of trust you have with your sexual partner(s), you are still opening yourself to more risk, the more partners you or your partner have.
And if you go into a relationship knowing that more sexual partners will be included in that relationship, you are simply increasing your risk.
It's just factual. Nothing personal - till it's you sitting with your feet in the stirrups being diagnosed with something rather nasty.