Food isn't important to someone who has never known what it is to truly go without food. Money doesn't matter to someone who is wealthy, a person who has never had money be limiter in terms of their options in life. To understand the human tendency to dismiss what we already possess as unimportant, you also have to understand the true nature of both pleasure and so-called positive value.
Have you noticed how food tends to taste so much better after even just a few days without it, or how water feels so good to drink after a day of hard work? On the other hand if one eats a consistently healthy amount, eliminating the gnawing hunger has less of a noticeable impact on them. If you compulsively overeat then you can only expect to at best alleviate some boredom, or to distract yourself from some other undesirable thing which is bothering you. My point is that what you're perceiving to be pleasurable isn't actually creating something positive, it's either reducing or eliminating an imposed need. It's a reduction in negative utility being perceived as the creation of positive utility, but ultimately there is no form of the latter which is independent from preexisting deprivation.
Another way to look at it is that we can clearly see a house to be a good thing, It provides shelter and warmth to the humans who inhabit it. But does a house continue being good if nobody exists to benefit from it's presence? I would say no, since there is nobody who has problems and needs which could be resolved by the house.
Now I'll get back to the topic of this thread. When people tell you that sex is unimportant, they usually aren't trying to lie to you. Because usually the people who make such claims don't know what it is to go an excessively long time with unfulfilled sexual desires, or to go without even much physical contact with other humans. To them sex maybe seems like a slight positive, a nice way to pass the time or to relieve a bit for stress caused by daily life. I'd wager that some don't even perceive sex as a positive at all, that it likely exists as just a neutral for some people, a totally given part of life, the presence of which brings no relief since it's so commonly had and expected.
To people such as this, the urgency which is common to so many of us isn't present at all, it's not something which they're perceiving. Sex doesn't matter to people who have a mostly or entirely fulfilled need for sex. The funny thing is, I've found that my own fixation with sex and my endless hunger for it mostly vanished after escortcelling a few times. I can only assume that it will eventually return, but for now I don't think about it so much, and it doesn't really bother me. My lack of good past experiences or positive responses from others has usurped sex as some of the primary things which bother my conscious mind. Which makes sense, since I have no immediate physical needs which require my attention.
The truth is we are need machines which will never be truly satisfied, pleasure is a measured reduction in negative utility, and people usually can't see a thing as valuable if they already possess it because they have no deprivation to relieve. If you see a depressed person in a first world country and you wonder why they're depressed because they don't have to fear starvation, then you're making a similar mistake. People who have all physical needs met kill themselves all the time, showing us that the presence of otherwise satisfied needs doesn't reduce the suffering one feels from unsatisfied needs, in fact it seems to do exactly the opposite.
Our existence is a mistake.