Why is vanilla called vanilla?

There's no other opinion than it's sex that involves no twists or kinkiness. Basically plain regular sex. Typically sweet and happy and very lovey-dovey. A cultural regard of standard or conventional sexual practice. Often, it is interpreted as sex which does not involve elements such as, BDSM, kink, or fetish activities. Usually defined as boring, plain sex. But, a vanilla lover would regard it as making-love, passionate sex.

Vanilla is called vanilla because it's plain or standard. Ever heard of vanilla World of Warcraft? It's used to refer to WoW before any of the expansions. For some reason though vanilla in the hentaisphere has sort of taken on the meaning of romance and fuzzy feelings which is why there will be vanilla chapters with like, sex toys and stuff.

Vanilla is a widely-used pleasant flavour used in sweets, such as ice creams and cakes. People like the taste, and it's fairly recognisable as a flavour, so there's big demand for it. Likewise, vanilla sex is inoffensive and lovey-dovey, and people eat that shit up. No pun intended.

It's just the easiest comparison to make when it comes to consensual heterosexual sex with a happy ending. It's inoffensive, pleasant, widely-used as a genre, and everyone likes it.

It being a standard and inoffensive flavour, as mentioned previously, is also why the term is used to refer to an unmodified game client. "Vanilla Minecraft" for example, refers to a bog-standard Minecraft game client without resource packs or mods.

Vanilla isn't exactly a hentai exclusive term, it has become an adjective in recent years meaning "having no special or extra features," which I think describes the tag pretty well. There aren't any kinks or fetishes or anything like that, it's basically sex in the missionary position for the sole purpose of procreation

vanilla (n.)

1660s, "pod of the vanilla plant," from Spanish vainilla "vanilla plant," literally "little pod," diminutive of vaina "sheath," from Latin vagina "sheath of an ear of grain, hull of a plant" (see vagina). So called from the shape of the pods. European discovery 1521 by Hernando Cortes' soldiers on reconnaissance in southeastern Mexico. Meaning "flavoring extracted from the vanilla bean" is attested by 1728.

Adjectival meaning "conventional, of ordinary sexual preferences" is by 1970s, probably from the notion of whiteness and the common choice of vanilla ice cream; vanilla as figurative of a plain and conventional choice (without reference to sex) seems to date to the late 19c. as a noun, by 1940s (often plain vanilla) as an adjective.

per Etymology Online

Cause I don't think people want to call it basic and vanilla is something I think of as easy to understand like the flavor. Just vanilla everyone knows it and what to expect, maybe it eludes to what one expects and it sorta just stuck

I think it's because vanilla is the neutral flavoring used for many otherwise unflavored sweets. For example, vanilla ice cream and yogurt are generally the plain flavoring.