Every person finds some members of the opposite attractive whether you're in a relationship or not.
You can't justify your behaviour with this if you have committed yourself to a monogamous relationship.
A monogamous relationship is when two people decide to be sexually exclusive to each other - they will only kiss/have sex with each other - but also they won't disrespect one another - they won't say horrible things about them, tell everyone how they were in bed, sit on another guy's lap, put their hands down another guy's trousers for a 'joke' - these kinds of things are disrespectful.
Disrespectful means not caring and not taking into consideration your partner's feelings in your actions.
Flirting, in my definition, is showing sexual intent towards a member of the opposite sex and leading them on sexually. Heavy eye contact, touching, sexual innuendos, talking about things you'd like to do to each other, telling each other how hot you think each other is, teasing each other -- can all be elements of flirting.
Talking to guys you merely find attractive isn't flirting. The sexual component is a heavy part of it for me.
Essentially, it's emotional cheating and that's what you seem to be doing with this 'one guy' who you find attractive and you think is flirting with you to get in your knickers. You like the attention, like thinking of him in that way, you like thinking what he wants to do to you in bed.