Having sex is good for you. There is amole evidence of immune system benefits, reduced cancer rates, improved mental health outcomes.
Having emotionally healthy relationships is an important consideration, and this can happen within marriage. Though increasingly (in western europe at least among my peers) people are having long-term monogamous relationships without getting married (despite all the tax breaks).
However sex ed tends to ignore a lot of the relationship advice, or emotional health side of things and instead focuses exclusively on physical health. (I know there are others on this thread who would prefer to look at this from a spiritual perspective, but i don't know what spirit is, so i can't honestly advocate for it; i hope most of what you mean is covered by emotional, mental and physical health) But we know there is a brain-body connection, and you can't just talk about physical health in isolation. Sex (and other physical contact, like holding hands, hugging and kissing) triggers the release of hormones (mainly oxcytocin, dopamine and seratonin) these go on to affect the brain, and are an important part of forming emotional bonds between partners (and likewise between friends) - we also know from studies of breast feeding that babies and mothers who breast feed get considerable doses of oxcytocin - and this is understood to be an important part of mother-child bonding (though oxcytocin is often called the love hormone, i've seen studies directly linking it to trust, not love... I suspect it is useful in developing loving relationships, because they are usually built on trust, but i don't think that necessarily makes oxcytocin sufficient for love...)
I don't think we should talk about sex without talking about love; we should recognise that they are seperate, and how to protect ourselves emotionally as well as physically. Love and sec are seperate but related.
Also, imo, from a practical point of view, learning to have good sex requires practice - and usually with the same partner; everyone is different and likes different things. So i would recommend lots of sex with one person if you want to have a good sex life. Lots if practice, lots of communication of needs/wants, and lots of honesty.
I don't know how you can have a comprehensive sex ed curriculum without these elements.