To sustain our species? Of course. On an individual basis? Not at all. Some people choose to live celibate lives and are at peace with that decision. That doesn't mean that there aren't challenges and temptations. But the same can be said for sexually active people.
All that said, I believe that for humans sex is mainly about pleasure and solidifying pair bonds since we are able and interested in doing it even when there's no chance of conception.
Let's be frank, sex is awesome. Not everyone needs to do it but most people choose too. I think most people would feel incomplete without it.