Dating the flirt
When you start dating someone new, you don't necessarily know whether they're a flirt or not. Many times a person's inclination to flirt comes about at a later point. Perhaps when they know that they've won your heart, or simply when they're comfortable enough to be themselves, they start flirting. Not everyone feels the need to flirt with others when they're dating someone exclusively. However, many people do, and many of their partners don't like it. For people that are dating someone who tends to flirt with others of the opposite sex, it can provoke many feelings of discomfort within their partner like insecurity, jealousy, hurt, and anger.
Not all flirts are shady
Just because someone flirts when they're in a relationship doesn't mean that they're the type of person that will cheat or be unfaithful in any way. Having said that, flirting can go too far at times, and it can be hard to predict the other person's response, even when the flirt himself isn't a shady or cheating type. Despite knowing yourself and that you'd never cheat on anyone, flirting with the wrong person just once could lead to something bad. It's always important to think of the consequences before doing things, even things as seemingly harmless as flirting.
The flirting type
I've dated the flirting type before, and I can honestly say that even when you're confident in yourself, it can suck watching someone that you're "into" flirt with other people. When someone that you're dating exclusively starts to flirt with others behind your back or in front of you, it can hurt, and cause you discomfort. Why would you want to make someone that you love feel that way! A flirt usually tries to come up with their own set of logic as to why them flirting with others is acceptable. They usually do this when they're not that into you. Again, when someone is crazy about you, they want to make you happy, see your smile, and that sparkle in your eyes. Dating the flirting type is a personal choice, and you have to decide if that's the type of person that you want to spend the rest of your life with.
Being too friendly vs. flirting
There's a very fine line between being "too friendly" and being a flirt. I truly believe that when someone truly loves and adores you, YOU will be the only person that they flirt with. I believe that flirting with others when in a relationship means that something in the relationship is missing, and the flirt is feeling it. I've seen (supposed) happy, older couples, where one of the two was always flirting with others. For them, it was totally understood and accepted by those around the couple when friends or family would see the flirt in action. It was simply part of the flirt's character and charm. Having said that, there's usually some grain of hurt that the flirt's partner feels, and something, yes, something is definitely lacking in some physical, emotional, or intimate way between the couple. When someone is too friendly, it can get them into trouble with their partner, and they might even end up in the dog house!
Suck it up or call them out on their shit
You can do one of two things when your partner is a flirting machine. You might deal with your own discomfort and suck it up, likely because you don't want to come across as insecure, untrusting, or you want to prevent drama, or you can communicate your feelings. Basically, you need to speak up or forever hold your peace. If you don't communicate to your partner that it bothers you when they flirt with others, they might think one of a few different things. First off, they might think that they're getting you jealous, and that they're becoming more desirable to you, because you'll see others interested in them. Another thing that they might think is that you don't care, and it doesn't phase you. As well, they might be trying to stir up your emotions, because of some hurt that they feel. Instead of communicating those feelings to you, they flirt to provoke you. They might even be trying to test you (not cool). There could be many reasons that someone may flirt with others. Perhaps they're not getting the attention that they need. They might simply enjoy flirting, and view it as part of their personality. Many flirts will try and turn things around on you, making you out to be insecure, untrusting, etc., but that's simply not cool, and you shouldn't stand for that. Again, something is missing for them when they try to pull that card, and it might be time to reevaluate the relationship, when communication doesn't help.
When it comes to dating someone exclusively, I believe in keeping that relationship as pure as possible. Flirt with your partner, and only with your partner. Give your partner enough attention, to where they feel loved, and that love should reciprocate. Concentrate on giving love, and you'll get that same love back. If you don't, you need to communicate those feelings in a nice, but direct manner to your partner. If you're the "too friendly" type, you need to chill out with that, and gain some self control. There's no need to be too friendly to people of the opposite sex. You're just asking for trouble, even when you're not the shady type. Save those exciting, tempting flirts for your partner. It might even be a good time to bring up the importance of date night to your partner. Think of your partner, appreciate them, and give them the security to know that you're into them, and only them.
Honestly, if you're not "feeling it," whether it be the love, the passion, the chemistry, or whatever, it might be time to head for the door, and peace out. Having said that, communicate your feelings to your partner if you feel that something is lacking, and never, and I mean never leave them in the dark when you're having doubts! Give your partner a chance to recognize what they're doing (or not doing). Don't simply give up on something that is predominantly a healthy, and loving relationship, just because of something that wasn't ever communicated to them. Get off of your high horse and talk with your partner, without expecting that they should already know what your wants and needs are. Relationships take effort, love, and good communication. Let them know how your feeling, and if you're open to giving things a chance to improve.