I'm one of those women... though, in my case, I wasn't exactly leaving him willingly.
As Lunatic Faith mentioned, a lot of "sisters" have met men online and decided they were better off with them than with their staunchly JW husbands who browbeat them with the bible and insist that they never do enough, are never good enough, are not enough, period. We are lonely, depressed, have low self-esteem, and on top of it all, we are emotionally, mentally, and verbally beaten and our hearts are left to rot. When someone comes along and shows attention, understanding, lets us speak our minds and feelings without judgment, it's not hard to see where an attachment might be formed. Is it right? No, and I'm not making excuses for my behavior or anyone else's. But a flower withers and dies when it's crowded out and subjected to long periods of drought...only sunlight and water can revive it before it's too late. And sometimes a person comes along who is sun and water to a withered and parched soul.
My ex forgave me for my indescretions (they were not consumated, there was no physical adultery)...but then when he found out I didn't want to be a JW anymore and I was miserable, he kicked my ass out so fast it almost made my head spin. We separated, he gave me permission, no ENCOURAGED ME, to have a boyfriend knowing full well that I might go further than the JW's deem I should. But he gave me his blessing to date other men. For that, he should have been DF'ed...but when I did do a little more than just kissing, he divorced me and got remarried right away. Basically, he made me a subject for adultery by his lack of love and affection during our marriage, and then again when he gave me permission to date others...yet, the elders smiled on his new marriage and even offered up the Kingdom Hall for them to be married as soon as the ink dried on the divorce decree. To date, he has neither been DF'ed nor reproved and is in good standing in the congregation.