Sex is always a hot topic in Burrowgatory, where the matter is simultaneously complicated, and quite simple.
Physicality
Succubuns possess limited shapeshifting abilities. In addition to shifting between their bun and doll forms, they are able to freely alter their sexual characteristics—chest size, genitals, facial hair, body shape, etc. Some prefer to keep one set of sexual characteristics permanently, some will occasionally deviate from their norm, and some are constantly changing. While it is a common trend to see feminine or masculine characteristics grouped together, a succubun's sexual characteristics are a grab bag that can all be changed independently of one another.
Due to the unstable nature of succubuns' forms, a small number of succubuns experience an inability to alter sexual characteristics. Those who get stuck with features they wish to change will often turn to surgery and hormonal medication, which are easily accessible.
Gender
Due to the mutable nature of succubuns' sexual characteristics, their culture does not formally recognize "gender."
Succubun culture does have a concept of "feminine" and "masculine" imparted on them through their contact with demons, but these are simply groupings of traits that individuals may choose to identify with. A succubun may refer to themself as a man or a woman in the same way they refer to themself as a goth, a knitter, or a bookworm—they are descriptive terms, rather than prescriptive.
Sexuality
Due to the lack of a gender binary in their culture, succubuns do not have a concept of being "gay" or "straight" in the same way humans do.
Rather than sexual orientation, they simply have preferences—some may have a specific preference for masculine, feminine, or androgynous partners, while others may have no preference at all for gendered traits. You may sometimes see a succubun who is exclusively interested in partners of a specific gender presentation, but preferences are typically more fluid.
Relationships
Rabbits are not monogamous creatures, and the influence of demons hardly made succubuns any more so. Most succubuns freely move between open relationships of varying levels of casualness, and even pairs with long-term bonds are generally expected to have other dalliances outside of their primary relationship. Hookups, friends with benefits, and short flings are common and encouraged by the culture.
The concept of marriage exists, but is uncommon, and takes the form of a personal promise of commitment rather than a governmental institution. Succubuns who have been each other's primary partners over many years may get married to highlight the importance of their relationship over any others they have, while an unhealthy relationship may see a possessive succubun insist on a marriage to "lay claim" to their partner. Though it's most common to have a single marriage at a time due to the difficulty of maintaining multiple "top priority" relationships, it's possible for a succubun to have multiple simultaneous marriages, or for a group of three or more succubuns to all marry each other. Divorce is fairly common, given succubuns' incredibly long lives.
A small number of succubuns are actually monogamous, desiring a long-term relationship with one partner where neither has any other relationships. These succubuns often form private special interest groups to meet like-minded people, as Burrowgatory at large finds their preferences strange.
Sex
One of Burrowgatory's favorite pastimes, sex bears no stigma to succubuns, who view it simply as one of life's many pleasures that they're meant to indulge in to their hearts' content. Virginity is extremely uncommon, as succubuns possess a higher average sex drive than humans, and most experiment with one another during adolescence. Succubuns cannot contract any sexually transmitted diseases, and pregnancy is only possible during mating season, making sex a low-risk activity.
Brothels, love hotels, sex toy shops, porn studios, strip clubs, and kink dungeons are all common sights in Burrowgatory, and attending or working at such places is considered perfectly respectable. While succubuns are very open about having sex, it's still frowned upon to have sex in public or places not intended for it—not everyone enjoys a peep show, or the cleanup. Dolls in skimpy clothing and partial nudity are common in Burrowgatory, but exposed genitals are considered impolite in public.
Sexual acts are most commonly performed in doll form, due to their humanoid bodies possessing a greater range of pleasure and physical capabilities. Dolls and buns are not compatible with one another.
A small number of succubuns may be asexual or have a low sex drive, causing them to be uninterested in Burrowgatory's sexual culture. While this is considered unusual and may be hard for some to understand, it's not stigmatized, so long as the succubun in question has other vices—if a succubun is indulging in the pleasures they want to indulge in, no one will question them choosing not to partake in others.