Intimacy is a natural part of our spousal lives as human beings, and having a healthy sexual relationship leads to strengthening of a marriage. It is not only the fulfilling of halal lust, but also contributes to avoiding of sin, which is why there are numerous Islamic rulings related to spousal intimacy between the husband and wife. Among these rulings is the obligation of 'tamkin' from the wife's side, which means her sexual availablility for the husband.
Both the husband and wife should facilitate to one another their sexual pleasures, within the rational and shar'i boundaries that are clear and obvious for any believer. The correct approach for this issue is to make sure that emotional and physical needs are fulfilled, and that it is not taken as a means for coercing the wife to accommodate to the husband at times beyond her capacity.
Therefore, in normal circumstances, and if there is no legitimate excuse, if the wife refuses to accommodate to her husband, she will be committing a sin, and this will lead to nushuz.
Of course, if there is coercion, then it means they are not at a level of correct communication, and that they have underlaying problems that they must address.
The wife must be considerate of this issue, and the husband must also be more considerate and cater to the wife's mood, or her being tired, or anything else that will not bring about any strain to the relationship.
And Allah knows best.