heres the deal...
chasing after someone who is taken is typically caused by the fear of intimacy.
may sound strange at first, but its actually not.
in a normal relationship the process typically includes an initial stage of judgement (each one of you judging the other).
if you make it through that then comes the process of trust where each of you begin to open up to each other. this stage if rife with people getting their hearts ripped out. its bad getting rejected in the initial phase when you hardly know each other, but to get rejected AFTER you start to open up is much more painful - becuase your authentic self is getting rejected.
but assuming you make it through this phase, then expectations enter the picture. so you're done judging each other, you now trust each other, now can each of you fit in to the future expectations of the other.
this 'normal' process can be TOUGH! at the same time it has pretty big rewards if you make it through it with someone.
now compare that process to chasing after someone who is already taken.
in this scenario the variables are completely different...
- the judgement phase is nowhere near as palpable, as it typically is centered on sexual attraction. so your personality and overall state of your being aren't really being assessed, nor are you assessing the other persons. you're simply enjoying the state of pure sexual attraction. because of this, this phase hardly has any judgement in it at all, in fact the complete opposite, its pure adoration.
- assuming you actually start seeing the attached person in question. the second phase isn't one of building trust, but rather one of meeting each others needs unconditionally. after all, how can you really trust someone who you know is cheating. this stage becomes purely about unconditionally satisfying each others needs and desires.
- assuming that goes on for a while you'll enter the third stage - the ultimatum. after playing lovers and it feeling pretty good, you'll then want to make it for 'real'. this is usually where everything goes to sh*t as you try to turn this 'fantasy'-based relationship into something real. suddenly all the warts and negative elements of the other person become visible.
now some people stay in stage 2 for years. some move to stage 3 and somehow work it out. most however hit stage 3 and it goes to sh*t.
but to your question of why some women chase unavailable men. basically they fear being hurt by a man who is looking for a committed relationship. they don't want to go through the processes required for that. you can also add to that... because they aren't the one cheating, they instantly hold the moral high ground on the person they are screwing, which further protects them from any emotional vulnerability that occurs during the normal courtship process.
any woman chasing after a man who isn't available is doing so because deep down she wants to 'act' out the play of courtship without really being vulnerable and without having to really commit herself to someone. the same thing holds true for men.
any one who truly wanted a mature, kind and caring relationship wouldn't 1) try to steal someone elses man and 2) wouldn't want a man that cheated on his gf / wife.
anyway, just my 2 cents.