If they flirt with and show an interest in other girls then they obviously don't respect their relationship, so what's the point of being in one?
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Same can be applied to women as well though. But to answer you question, maybe because they aren't really into the whole "committed" part of a common relationship and, for example, see it just as more stable route for getting sex. But then again, just flirting with another person doesn't make that person a side anything and shows of interest is a rather vague statement. I mean someone might think one action is a show of interest and another might not.