There is at least one exception to religions talking about sex in a mostly negative way: The Our Whole Lives curriculum, which is developed jointly by the Unitarian Universalist Association and the United Church of Christ.
Our Whole Lives is a series of sexuality education curricula for six age groups: grades K-1, grades 4-6, grades 7-9, grades 10-12, young adults (ages 18-35), and adults.
Our Whole Lives helps participants make informed and responsible decisions about their sexual health and behavior. It equips participants with accurate, age-appropriate information in six subject areas: human development, relationships, personal skills, sexual behavior, sexual health, and society and culture. Grounded in a holistic view of sexuality, Our Whole Lives not only provides facts about anatomy and human development, but also helps participants clarify their values, build interpersonal skills, and understand the spiritual, emotional, and social aspects of sexuality.
I had a pretty informative sex ed class in public school, but it was peanuts compared to OWL, which I've taken as a teen and as a young adult. The core sexuality curriculum is very inclusive and sex-positive, while the spiritual values are framed in (liberally-interpreted) Biblical teachings for UCC, and humanist philosophy for UU. Happy Mutants will probably prefer the latter, and churches are usually open to non-members taking OWL classes.