Extramarital Flirtation Is Not Inherently Sinful

That certain forms of polyamory are not Biblically sinful is quite evident both from the instruction to not add to God's commands (Deuteronomy 4:2) and the fact that miscellaneous parts of God's moral revelation codify polygamy (and polyandry by logical extension)--something a morally perfect deity could not do if polyamory is inherently wrong (James 1:13).  This is a more widely-known truth about the Bible than the point I want to make here, ironically: it is not immoral for a husband or wife in a monogamous marriage to openly flirt with other members of the opposite gender.

One of the largest potential objections to extramarital flirtation within the evangelical mindset treats flirting in this context as an adulterous betrayal of one's romantic partner, or at least a precursor to adultery.  Those who adhere to this position do not understand either flirtation or adultery.  Flirtation is not inherently adulterous because it may have nothing to do with adultery.  Moreover, flirtation has no inherent connection with sexual expression as a whole at all, and this undermines any attempt to pretend like extra-marital flirtation is even romantic or sexual to begin with.

Flirting might occur with or without sexual or romantic attraction as a motivator, and, since intent is what defines something as flirting in the first place, it can look very different when different people engage in it.  Whether a person is comfortable with their dating partner or spouse flirting with other members of the opposite gender is a subjective matter of no moral consequence, and thus has nothing to do with whether the Bible truly opposes extramarital flirtation.  Just as the Bible does not condemn polyamorous relationships (although they are certainly not for everyone), it does not condemn the lesser act of flirting with multiple people while married, sexually or not.

Of course, not everyone is upset when their partner does such a thing, so the idea that flirting with random strangers or certain opposite gender friends is inherently offensive to one's spouse, even when it is not motivated by a desire to experience a rush of sexual pleasure, is nonsense.  Since no one can have a right to demand that another person abstain from a nonsinful thing, to fiercely object to this in itself can reek of pettiness, legalism, and irrationality.  Flirtation can be a fun, simple way to feel good or to express innocent sexual attraction, which is not Biblical lust [1], to someone besides one's spouse.

Extramarital flirtation itself is not Biblically wrong.  Flirting to intentionally upset a spouse and flirtation motivated by actual lust are immoral, of course, but behaving or speaking like one is attracted to someone of the opposite gender, even if sexual or romantic attraction truly is beneath the surface, is not inherently sinful.  Not only can casual flirtation simply be a pleasurable, innocent pastime, but it can also provide practice for the flirting that could occur in one's romantic relationship, whether or not one is already in such a relationship.  Thus, it can be enjoyable and pragmatically beneficial to have past experience with flirtatious relationships or even to have friends or acquaintences with whom flirts.

Subjective pleasure and pragmatic benefits are of no concern if something is immoral, of course, yet the Bible is not ultimately opposed to flirting with random people of the opposite gender after one is married.  One can demonstrate that the Bible does not condemn extramarital flirtation in the same way one can demonstrate that it does not condemn profanity, nudity, mere alcohol use, or playing video games: it demands that readers not add to its moral commands in multiple places (like the aforementioned Deuteronomy 4:2) while never mentioning extramarital flirtation or revealing any moral concept from which it follows that extramarital flirtation is sinful.

[1].  https://thechristianrationalist.blogspot.com/2017/12/the-truth-about-erotic-media-part-1.html