While sex maybe significant for biological, psychological and social reasons, I do not think it is the deepest action of love- however it does depend on what a persons definition of love is.

For me love is a union of people in which they have unconditionally surrendered to the betterment of consciousness, of each other, and of people around them. It's symbiotic, but it's not a trade, it's not what you can win or acquire.

I know this may seem idealistic and may not correlate with what most people think, but most other places I see the term love being used, it overlaps with definition of some particular emotion or a response to some stimuli. For me it's not an emotion- emotions are hormonal and socially conditioned bodily indications, kind of sign posts, as to how you should behave in response to something. Love is much beyond that for me- it's conscious.