How people manage multiple, concurrent romantic and sexual partners may influence sexual health and wellbeing. We examined whether these outcomes differ between monogamous, consensually non-monogamous, and infidelity (i.e., non-consensually non-monogamous) relationships in a multinational online sample (Study 1; N = 888) and Finnish population-based data (Study 2; N = 10,503). In Study 1, women with multiple partners reported greater desire and satisfaction with their non-primary partners (vs. primary partners), and greater arousal with non-primary partners compared to monogamous women with their partners. Polyamorous individuals were more likely to have ever been tested for sexually transmitted infections but not more likely to have been diagnosed with one. In Study 2, non-monogamous men reported fewer early ejaculatory symptoms, and non-monogamous women reported higher sexual desire than monogamous participants. Both men and women in non-monogamous relationships reported greater desire for partnered sex. Across both studies, sexual functioning was largely similar between monogamous and non-monogamous participants. Infidelity was not associated with sexual health, apart from men who reported keeping emotional or sexual intimacies secret from their partners also reporting lower sexual satisfaction (Study 1). The results offer evidence that having multiple partners, whether consensual or not, may not necessarily compromise sexual health. Rather, men’s secrecy about extradyadic partners may be reflected in sexual dissatisfaction, but this effect does not extend to physiological or desire-based measures of sexual wellbeing.