Introverted sensing is probably the function related to sex
So depending on where it is in a type's functional hierarchy, it would predict their skill & attitude towards it
Base - Naturally good at sex, able to 'lure' a partner by adjusting their appearance and body language, as well as the external environment. Confident in their attractiveness and those of others. Sex is preceded by elaborate foreplay, base constantly attunes their movements to their partner's to keep both in harmony. Able to feel the other's pleasure as their own, and instinctively knows how to prolong pleasure over time.
Additionally, easily repels unwanted sexual advances by creating waves of discomfort (related to restrictive)
Creative - Creative in sex, able to relax and sensually arouse practically anyone. Partner's desires take priority over their own. sexual activity is dependent on their base however, and not given the importance of base, and so one could go long periods without it.
Role - Sensitive to anything that could cause discomfort, such as bad smells, poor hygiene, unusual tone of voice. Uncertain of their own sexual appeal, needs a partner to confidently confirm it and be able to take the lead. Quite indulgent at times, though more likely to suffer from self imposed shame.
POLR - Sexually picky. Needs a partner to conform to an inner ideal, will not have sex with just anyone. Sensitive to manifestations of discomfort in their partner but has no natural ability to ease it, expects the partner to adapt to them. Will be quite restless and uneasy without this adaptation. Real sensations at risk of becoming subordinated to imagination, so they sometimes lose touch with their true sensual preferences and those of their partner.
Suggestive - Hesitates to initiate sex unless partner is clearly receptive. Often oblivious to their partner's discomfort and their own, can unintentionally attract or repel a partner by their mannerisms, clothing choices, etc. Evasive of strong sexual advances, but receptive to gentleness, needs a lenient partner who can go at their sometimes 'staccato' pace. Sex takes a backseat to intellectual pursuits so is sometimes sexually inert.
Mobilizing - Needs to be certain about their sexual attractiveness or suffers self esteem issues. Trusts only time tested feelings of sexual desire, and can become possessive of partner. Often romantic and creative during sex, attempts to create an alluring atmosphere, resentful when this is not appreciated or reciprocated. Sexual preferences sometimes unique, unaligned with social norms.
Restrictive - Confident of themselves and their right to sex, able to easily initiate sex when desired. Not deterred by the hesitation of their partner, will pursue them. Strongly aware of their sexual preferences and will insist on them to their partner. Sex is viewed as a natural part of life, and a way to release their considerable volitional energy
Demonstrative - Sexually robust, with the right partner can have passionate intense sex for a long time. Easily and unconsciously conforms to the partner's sexual ideal. In periods between sex maintains an unconscious receptivity to it.
edit: PROBABLY related to sex, reiterating that for everyone's attention. i do believe introverted sensing is involved in sex but the details of what ive written here are mostly speculation, ie pure guesswork really. if your own experiences contradict them feel free to oppose.
as for why is involved, i kind of think the introverted/extroverted functions work in tandem and that goes for every pair not just se/si and not just in sex. its like one provides the background for the other, attention flips between both