Polyamory requires communication and consent, a healthy polycule is no different than a healthy monogamous relationship when it comes to the expectation that your partner(s) will care for their sexual health and make decisions with you in mind. In one way you are correct, more people involved means an increased possibility that someone will be dishonest or take risks that will expose you. However, in the same way that bringing an STD into a monogamous relationship has consequences so would bringing an STD into a polyamorous relationship. You risk damaging your standing in the relationship.