My husband seeks online relationships with different women.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Having affairs with women over Internet or by any other means is forbidden.  So, one must give up this practice and make sincere repentance.

It is the wife’s duty to exhort her husband, advise him not to commit such a sin, and warn him of the punishment of Allah.  She should also make Du’a for his steadfastness on the right path especially during the times when Allah accepts the supplication.   The times when Du’a will probably be responded to are the last third of the night, during prostration, and after the obligatory prayers. If your husband does not give up that practice, then you have the right to ask for divorce from him.

This advice is on the assumption that the relations you mentioned are illicit. If he wants to marry another woman and takes the legal steps to do so, then you have no right to object to that and should not ask divorce due to this reason.

Allah has permitted him to marry more than one wife, Allah Says (interpretation of meaning): {…then marry (other) women of your choice, two or three, or four ….}[4:3].

But, if he really wants to marry another woman he has to contact the guardian of the woman, not the woman herself.