Don't people need sex?
The short answer is no. If someone tells you they're fine not having sex, you should just believe them. Some studies have found correlations between sexual activity and health (physical or mental), but it isn't known what the causal relationship is in that situation, nor whether such findings apply to asexuals in particular.
Maslow's hierarchy of needs is a popular framework that sometimes places sex as a base need. However, in academia Maslow's hierarchy has always been contentious – in particular the position of sex in the hierarchy has been heavily disputed from the very beginning. In any case, Maslow's hierarchy hasn't made any successful predictions, so it isn't really a scientific theory in the normal sense.
Of course, while not everybody needs sex, it is still valid that some people do. (In this case, we're using 'need' in the sense of 'I need space', etc.) Different people have different needs, that's entirely normal.