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?
Most Helpful Guy
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.0