Small studies suggest significant differences between autistic and nonautistic individuals regarding sexual orientation and behavior. We administered an anonymized, online survey to n = 2386 adults (n = 1183 autistic) aged 16-90 years to describe sexual activity, risk of sexually transmitted infections (STIs), and sexual orientation. Autistic individuals are less likely to report sexually activity or heterosexuality compared to nonautistic individuals, but more likely to self-report asexuality or an 'other' sexuality. Overall, autistic, and nonautistic groups did not differ in age of sexual activity onset or contraction of STIs. When evaluating sex differences, autistic males are uniquely more likely to be bisexual (compared to nonautistic males); conversely, autistic females are uniquely more likely to be homosexual (compared to nonautistic females). Thus, both autistic males and females may express a wider range of sexual orientations in different sex-specific patterns than general population peers. When comparing autistic males and females directly, females are more likely to have diverse sexual orientations (except for homosexuality) and engage in sexual activity, are less likely to identify as heterosexual, and have a lower mean age at which they first begin engaging in sexual activity. This is the largest study of sexual orientation of autistic adults. Sexual education and sexual health screenings of all children, adolescents, and adults (including autistic individuals) must remain priorities; healthcare professionals should use language that affirms a diversity of sexual orientations and supports autistic individuals who may have increased risks (affecting mental health, physical health, and healthcare quality) due to stress and discrimination from this intersectionality. LAY SUMMARY: This is the largest study on the sexual activity, orientation, and health of autistic adults. This study reaffirms that the majority of autistic adults are interested in sexual relationships and engage in sexual activity. Sexual education and sexual health screenings must remain a priority for all individuals, including those with autism; healthcare professionals should be aware that autistic patients may be more likely to identify as Lesbian, Gay, Bisexual, Trans, Queer, Asexual, and other identities not listed here (LGBTQA+) which may put them at greater risk of mental and physical health difficulties due to discrimination.