When a man and a woman marry each other, they agree to an unwritten rule of taking care of each others sexual needs, cause any other need can be fulfilled by any other person but sexual, so sex with spouse remains the main obligation of married people.
If a person keeps on rejecting that need of his or her spouse, then the person is disrespecting the spouse. Every now and then, I am sure that any decent man would back off when realizes that wife is tired, or not in mood etc. But if wife is continuously refusing the requests then the man would be left with threes options.
- Non consensual sex with his wife
- Go and cheat on her with another woman.
- Divorce her and marry another woman.
Wife has to agree with one of them.
Martial sex is a bull crap term probably coined by greedy lawyers or sick minded journalist. If Islam does not recognize it, then there is a reason. Wife is not a victim, cause she is not locked in a jail or has been kidnapped by the husband. Under any normal circumstance, she can leave him if she is not happy. There are no obligations on her to have sex with him, but then she should not remain his wife.