As a 16 year old male, it seems stereotypical of me to enjoy the idea of sex, I've never had it and perhaps that voids my opinion on this subject a little, however, I've been in a relationship for near on 2 years now with a girl whom I love dearly. As my relationship has progressed, my views on sex have changed quite considerably.
At the present time, I believe that sex is very much something that should be shared with someone you love and I believe it's also an important part of a long term relationship because, as other posts have stated, its something that you can share as part of the intimacy in your relationship. In contrast of this however, as part of being with my girlfriend, who has a stronger religious background than myself and believes in no sex before marriage (a view which I whole-heartedly respect), I've learnt the joys of sharing intimacy and love for another person without going "as far" as sex.
Overall I'd say that clearly the value of sex varies from person from person (hence this thread) but I feel it would be something that's significant and important to a long term relationship and love.