quite often people ask that a particular act done by the life partner is a cruel act or not and whether it entitles him/her to seek divorce. The courts often said in unambiguous terms that "cruelty" cannot be defined in precise terms. The parties upbringing, culture, social and economic back ground will be taken into consideration to come to a definete conclusion whether such act is cruel act or not. The courts have never set a list of acts to define them as cruel acts, which entitle the party to get divorce decree. Calling the wife 'b*tch' in certain circumstances will be curelty and in different set of circumstances, it is not so. similarly calling husband 'b*st*rd' may be cruelty in certain family cultures and not so in other family cultures. so, do not attempt to come to a final decision on the basis of precedents or other wise to define the act of "throwing mangal sutra" is a cruel act or not.
If I am not wrong, in the case of author, (on the basis of his other postings), hiswife has not only thrown the mangalsutra but also insulted him by calling him "hijra" If I am wrong, kindly ignore this remaining part of my answer. After receiving such an abuse, he left the wife and started to live with his parents and does not want to have any physical relationship with his wife.
Now, we will take this case in two ways. If he is really potent and is put off due to that woman's odd and atrocious behaviour, then throwing mangalsutra coupled with calling him so, are certainly cruel acts.
Now, if the wife's allegation is correct and he is really not competent to have physical relationship, then it is not a cruel act.
But, we must understand one thing. Being impotent of incurable nature is one of the disabilities. If a person is disabled due to blindness, deafness, dumbness, loss of limbs etc. we as a whole society shall shower pity and try to help them, whenever we come into contact with them. But in the case of impotency, we completely adopt a most hateful attitude. Being impotent is not a sin. He/she does not deserve ridicule from our part. Compassionate attitude is the need of hour. If a impotent person (either girl or boy) is stuck in a marriage and if it becomes a burden to her/him to carry such relationship, the other partner must allow the troubled spouse to get free of matrimonial bond.