A priest told me that there is a "grave obligation" for a husband and wife to have marital relations and that a refusal by one spouse to engage in marital relations could be justified only by "grave reasons." Is this true?

In marriage, the husband and wife have become one flesh. They are no longer totally independent. By their wedding vows they have given the right to their own bodies to each other. So yes, there is a grave obligation that each has to be sexually available to the other at appropriate times and in the context of generosity and mutual consideration.

There are times when, for any number of reasons (including health), marital relations would be painful or burdensome. Concern for the other ought to be the norm on the part of both parties.

If either party has a continuing aversion to marital relations, medical and/or psychological help should be sought.