Does the husband and wife have a serious obligation to marital relations??
(there are exceptions and more to be said here on this -such as spouses can agree (when there is not a near occasion of mortal sin) to postpone or such as when it can be a lighter sin or when the obligation is not there (like the person is drunk or has committed adultery etc) or when there is no sin etc)
Yes. Most certainly.
catholicreference.net/index.cfm?id=32752
Can there be sin within the sexual relations of husband and wife? Yes.
Can there be the sin Adultery per se? No. Is that sin Fornication ? No.
Why not? because they are married to each other.
They can only engage in "adultery per se " with someone who is not their spouse.
(There be a kind of “adultery of heart” but it is not per se “adultery” in the sense of having relations with another while one is married or they are married)
They can only engage in fornication – PRIOR to marriage. Even relations with another person here would not be “fornication” it would be “adultery” (which is much worse and would need to be confessed as such).
But there can be sin within the sexual relations of husband and wife --and indeed within the marital relations. There can be sins.
Now we come to the question of say a husband physically forcing his wife to have relations. Such would be rather not “marital” relations. But such would not be called here I think (along with St. Thomas and others) “rape” -just as it would not be called “fornication”. Except perhaps by “analogy”.
This is getting into what “term” to use -not in anyway making light of the evil of this sort of action --which should NEVER take place.
It is an act of sexual violence or sexual assault committed against one spouse! Against the one whom one has promised before God to love and honor …the one whom one is to have given oneself to in love and family…an act of violence against one that one is joined in a sacrament with --a form one could say argue even perhaps of “desecration” of what should be the “marital act” of self gift …of living out the self gift already given in the rite of marriage…and is not. Such is not a “marital act” but an act of sexual violence/assault against ones very spouse. Such could be argued I think to be often worse than “rape”.