To me sex is sacred, someone you share with the person who will discover you and an experience worth remembering. (mine)
I agree with your view that there are way to many derogatory views about sex, but if in relation to polyamory that's a different matter.
You do not need to have sex in a polyamorous relationship. If you just want to be with someone other than your main Girl or Boy, someone to chat with that's ok too (we call that friendship). But if your main reason in a polyamory relationship is just sex, ^^, that's something you will have to figure out on your own.
Sex in general is wonderful, it's a good exercise, fun, adventurous and very personal.
It is no way a sin, I agree with your view that it is not, the organized religion did that so that no one will be able to get away from sin. Smart kids, really >,<
Sex with multiple partners => always have a regular check up. >,<
Peace, safe sex guys. haha