Well, sexual education is much needed. If it is too much taboo talking about pleasure, ok, just give it a "public health" focus and there will be tons of topics to be explained forever: diseases, pregnancy & responsibility, hormonal maturity, etc.
I believe that, exposed in an honest, broad, natural and principally open
way, this could even mild a bit of the mystery and prohibition aura that makes sex so interesting to premature youngsters.
If the approach is broad enough, it will contemplate all cultural / religious backgrounds by mentioning them.
"Ok, people can start their sex life artificially too early, they can start when they naturally feel like regardless any background, they can respect their background and wait for a cultural/religious symbolical moment, such as marriage or adulthood passage rite, etc...
They can then do sex for pleasure, they can refrain from doing it and only use it as a reproduction tool, they can use it as a day-to-day exercise for energy and disposition, etc.
When doing sex, they may prevent pregnancy by doing this and that, or seek pregnancy by doing that and this, but there are DSTs, the matter or trust, etc etc"
... and so on. If it isn't broad enough, sexual teaching should be open and inviting enough.
Diversity, richness, rather than obscurity. Nobody will be forcing students into some specific way, but keeping them well informed in whichever way they choose.
The very, very same goes for religious studies : if it is to be approached, so make it universal, mention from the African Umbanda to the esquimos spiritual belief to Greek Mythology to Buddhism to the hardcore-Catholic Opus Dei. Even if there's only time to scratch the surface and leave rich homework up to the student's interest.
Here in Brazil, "Religious studies" usually mean "One more way to interpret the Bible". Ok, Bible, cool, but DANG, what about the whole humankind religious experience? The Coran, the Mahabharata, the Brazilian Indigenous mythology - at least cite their existence, let me have my insight, make my personal harvest!!
Otherwise, I dare say, it would be better to leave sex and religion unmentioned at school. I really don't see a mid-way that doesn't sound poor or somehow disrespectful.
Horny holy Cheers!