It seems to me that sex is used for a variety of purposes by humans. For instance:
1) Reproduction.
2) Bonding.
3) Reconciliation, such as after a disagreement or fight.
4) To express dominance. At least, in some animals, one way dominance is expressed is through sex and I wouldn't be surprised if something along the same lines doesn't happen now and then in humans.
5) Consolation. Seems to me it's frequently used to console people.
6) Entertainment. The importance of which should perhaps not be discounted.
7) And perhaps miscellaneous other uses, as well.
Now, it does not seem possible to me to objectively rank the importance of these uses. Any ranking of them must to some extent be subjective. Even if you were to say that reproduction is the most important use because it leads to the survival of the species, you are making a subjective judgment that the survival of the species is more important than, say, any of the other purposes of sex in humans.
But what do you think? Are any of these purposes or uses for sex objectively more important than any others? Why or why not?