Some people wonder why married people flirt. They make the false assumption that the taking of vows suddenly turns off any playfulness or sexual attraction with anyone but a spouse.
Flirting Is a Release Valve
Married people who realize that flirting can be a release valve may also realize that there is a lot of pressure when you try to just "turn off" what might have been an ingrained habit established over years or decades. Not that everyone needs to flirt, or that everyone flirts, but if the reason you fell in love with a girl in the first place is because she was outgoing, teasing, and playfully engaging with you and others, why would you think that would be easy to just turn off?
Old Habits Die Hard
Newlyweds especially usually only have eyes for each other, which makes it seem easy. But once that new-relationship energy wears off, and they settle back into their everyday life, the original habits return - and having to struggle to turn them off can become very frustrating. One of the reasons why married people flirt is simply to keep that fun in their lives - so that being married doesn't mean losing a fun and harmless part of their joy in life.
Flirting is how some people relate to others. They don't see it as an invitation to intimacy but rather as a means of communication. For these folks, flirting isn't disrespectful to their spouse since nothing naughty is intended. In this case, the flirting is harmless and innocent.
Married Flirting Is a Matter of Degree
Of course, before married people flirt, it's probably a good idea if they work out with each other what the definition of flirting is. Some very insecure and jealous people will try to enforce a rule like "Don't even look at another person!" which is both unrealistic and unfair in a culture that counts on sexy forms in just about every kind of marketing.
Some married people flirt solely to reassure themselves that they still have the ability to attract people. For most married people, this flirting takes place with no intention of seeking an affair but instead so they can walk away from a flirty exchange of words feeling like "they still have it."
It's a good idea to talk out with your partner what you think is flirting and what isn't. Friendly conversation? Dancing the tango? Going to coffee? Sexual innuendo? What about online chats? All of these could be considered a form of flirting and knowing what level it becomes uncomfortable for your partner lets you both make informed decisions - whether that be modifying your own behavior or your partner working on handling insecurity and jealousy.
Boredom in the Relationship
Marriage takes work, and the monotony of everyday life can make married people forget to flirt with one another. When a person needs the thrill of flirting and doesn't receive it from their spouse - or if for whatever reason they're bored by the flirting provided by their spouse - they may turn elsewhere for that thrill. While there is no harm in this behavior (providing it doesn't lead to other things) it can certainly be indicative of a need for better communication and maybe spicing things up a bit.
Breaking the Rules
Of course, if married people flirt with others, it's also usually a good idea that they flirt with each other, as well. One of the biggest dangers of marriage is reaching a point where it no longer feels exciting where you feel like you're in a rut. Some people go so far as to fear that they've fallen out of love with their spouse, and that's when rules and marriages get broken.
Flirting With One Another
Often what is really happening is a transition into a new kind of relationship, that goes deeper than the hunt-and-chase of the flirtatious dating scene and into the realm of security, trust, and commitment. But dating is fun, and by making sure you and your spouse are both still flirting, teasing, and dating - even after decades of marriage.
Words vs. Nature
The idea with marriage is that saying "I do" usually includes the unspoken understanding "... and I won't with anyone else!" The idea of fidelity, of "cleaving to you only" is integral and common to most marriage vows. That kind of action is well within the power of the husband and wife - you can always control your actions. What you can't control is your desire - that is, you can't say "...and I won't ever want to with anyone else!" If a person has always been turned on by firemen, for example, the idea that saying the words "I do" will suddenly turn off that biochemical response is ludicrous.