Talking about sex with friends isn't taboo.
My point is, at what depth? It might not be (in my circles it was) to brag about having sex, but think about all the other sides around it, the things that matter - do people dive deep into that?
I discuss investments and business ideas with my friends a lot of times.
Again, the circle is very small, isn't it? Should it be? I don't think so. This "small circle" is what makes it taboo.
I don't know about you, but the circle isn't small in my case. I'm introverted, and the number of people I interact with might be considered small, but I don't think I've encountered a situation where it was taboo. Most, if not all, of the people I've communicated with have been very interested when we talk about investments, business, and money. If it's about making more money, I haven't met anyone that is against it. My previous coworkers bought crypto, and are still updating our chat when it goes up. Another coworker bought stocks, and have been asking for tips and suggestions. My friend has been discussing about his plans on opening a small crepe shop, and wants me to try some of the menu.
By things that matter are you referring to these: "... the ideals, the human needs for intimacy, the desire that we have to be known to others, to be part of a community, a society and to feel relevant"? Honestly, it is arguable if these can be considered in the same level as sex; I don't. A lot of these, for me, fall into the category of Love. But either way, I still don't consider them as taboo, but rather, people are hesitant to talk about these because they make people feel vulnerable. A lot of these tackle the deep insecurities of a person, their inner workings, and their weaknesses. It is not that it is taboo, people just choose who they want to be vulnerable with.
Try discussing these things with strangers, then see if it is taboo? Taboo isn't about discussing things with friends, it is about the culture of discourse.
If you discuss these with strangers, I see a lot of replies possible. If you discuss money, they will think you're trying to scam them. If you look legit, people will listen. I bought an investment linked insurance inside a bank from a stranger. I wasn't there looking for one, but I liked the pitch. I also sold an investment linked insurance to a stranger. Like I said, it is looking for the right audience.
If you want to talk about sex with strangers, that is just crazy. One because that is just too much info [TMI]. Would you talk about your diarrhea to strangers? Another reason is, why would strangers care about your intimacy problems or ideals. They don't know you, they have better things to do. If you go to an art community and want to be part of that community, they will welcome you with open arms. If you go to a gym and ask the stranger veterans about your fitness goals and ideal body, a lot of them will be happy to help. Like I said, it is about knowing the audience.