Baba Kama 89a - Keeping a Wife without a kesuba

R' Meir says that it is prohibited for a man to stay married to his wife unless she has a kesubah. The purpose of the kesubah is to prevent him from divorcing her hastily. Being that upon divorce he will be required to pay a significant sum of money, he will likely be deterred from the divorce. The gemara is clear that this takana does not require that she be the receiver of the kesuba, because even if she were to sell the kesuba and someone else would be receiving it, the fact that he would have to pay someone would still deter divorce.

The Shulchan Aruch (66:3) paskens that if a woman looses or forgives her kesubah, she cannot continue to live with her husband until he writes her another one due to this takana. However, the Rama writes that the entire concern is only at a time when there is no other form of protection to prevent divorce, but nowadays where their is a cherem of rabbeinu gershom that prevents him from divorcing her against her will, it is not so vital that there be a kesubah. Although the rama says the minhag is not to rely on this, he indicates that me'ikar hadin, it would suffice. The chelkas michokek, beis shmuel and gr"a are all somewhat skeptical about the validity of this heter. We do find that when one rapes a woman and there is an issur d'oraysa preventing the divorce, one can be lax about the kesubah since he won't divorce her "easily". But, who says that a cherem is enough to fulfill this requirement! Furthermore, the Mishneh l'melech (ishus 10:10) proves from the Ran that even in a situation where it would be prohibited to divorce a woman against her will, a kesubah is still necessary. He proves this from the question that the Ran asks, how can a woman be mochel her kesuba, it will be a violation of living with a wife without a kesuba (tosafos by us asks the same question). From the fact that we don't give a very simple answer that when he swore not to divorce her against her will she can be mochel, implies that a kesubah is always necessary. Just as swearing can't replace the requirement of kesuba, so too the cherem of rabbeinu gershom cannot replace this requirement.

It seems to me that there is an even greater proof from our Tosafos against the Rama. Tosafos asks that when she damages her husband and owes him money, we should force her to sell her kesubah to a third party (to have money to pay the damages she owes him) and he would sign on the contract to prevent her from being mochel. Tosafos answers that by the husband signing directly to the third party, it would no longer qualify as a kesubah and would be a violation of the din of being married to her without a kesubah. We see from Tosafos that the takana d'rabonon was not merely that the husband should have to pay a particular sum of money if he would divorce her, but rather it must be in the form of a kesubah that is owed to her (even if she sells it, it is going through her and qualifies as a kesubah). So too the cherem d'rabbeinu gershom which is another method of preventing divorce but not using the concept of a kesubah, does not fulfill the takanas chachamim.