In Catholicism, the conjugal moral of the marital act is coming from the natural institution and from the natural contract between spouses: that is the natural moral law, thus it is existing between wife and husband the rights with the corollaries, the duties, obligations (to do, not to do or to give). The rights, the obligations, the duties from the contract are not absolute, of course. Only a good, just, real, objective reason can modify the degree of the rights, the obligations, the duties from the contract.
The conjugal moral of union of bodies has two dimensions:
The natural moral order coming from the natural sexed and sexual institution: the respect of liberty, of life, of potential fecundity, of dignity of the human person, of equality between wife and husband, of the male potency, of the female potency, of the health (physical, psychological and physiological), of natural mechanisms of union of bodies…etc;
The natural moral order coming from the natural sexed and sexual contract: the respect of liberty, of dignity of the human person, of equality between wife and husband…etc. It is the special relationship between spouses, the conjugal personalism (the vow on the reciprocal and mutual donation of bodies, in canon law , the lawyers use the term of the right on the body).
The mixed nature (institution and contract) of marriage means that it is difficult to indentify the two dimensions in the conjugal life, day a day, because sometimes or often the actions can be in link with the two dimensions or one only. Thus, the type of moral rules, that have to be used for finding the balance, the moderation in the respect of conjugal moral, when there is a problem or great issues, is difficult.
The love is the first rule, of course. But there are the other rules.
I do not understand that many catholic couples are not in the situation to understand the totality of the catholic teaching concerning the marital act his mixed nature (from the institution and from the contract), his meanings, his functions, his ends, his corollaries, his obligations, his rights…etc. There is an unity of the catholic doctrine , but the fact of wanting to separate each part of the teaching is, in my opinion, a wrong approach. The notion of " conjugal debt " is existing for solving the issues, that is the ultimate mechanism of the conjugal moral.