So what is considered unjustly? One persons reasoning may not be considered just to another person, where is the line drawn?
That’s a big question: what is justice? There are entire courses in ethics and philosophy that discuss this question. I cannot provide a comprehensive answer here, but I can give some introductory info…
Something is unjust, first and foremost, if it contradicts the way of Christ. God gives us many “drawn lines” within the deposit of faith of Divine Revelation, which is entrusted and handed on in the tradition of the Church, both oral and written.
How one goes about informing their conscience regarding the Divine will, and then follow their informed conscience it is a bit more difficult to summarize.
Nonetheless, one thing to keep in mind is that one can sin by commission or by omission. A sin of commission is deliberately doing something evil. A sin of omission is a willful
neglect of doing something good, neglect of an obligation or duty.
With regard to marriage, the vows normatively imply giving of themselves to each other, to include conjugal relations.
Among other things, marriage is directed to the avoiding of sins against chastity (cf. 1 Cor. 7:2). Thus, marriage involves an obligation, and when one neglects an obligation, it is sin. This sin could be a stumbling block to one’s spouse, who may be increasiningly tempted to sin against chastity.
Can a spouse have trivial or “unjust” reasons for not having sex with their spouse? Sure. “I want to watch T.V., go away.” This kind of response to a spouse’s request for sexual relations may have the consequence of placing their spouse at risk of committing sins against chastity.
Can there be just reasons to either permanently or temporarily abstain from sexual relations? Certainly.
For example, the text
Contemporary Moral Theology,Vol. II, Marriage Questions, by John C. Ford, S.J. and Gerard Kelly, S.J., (Westminster Maryland: Newman Press, 1964), dedicates three chapters to the moral licitness of using periodic continence (i.e. periodic sexual abstinence) to avoid pregnancy. According to Fathers Ford and Kelly, there are three conditions for the morally licit use of periodic sexual abstinence (pg. 387-388):
(1) both parties were willing to do so (this meant that they were
mutually agreed, that neither party forced the practice on the other);
(2) both parties were able to do so (this meant that the practice
did not involve either one in the unjustifiable occasion of sin, for
instance solitary sin *, or other sins against chastity, etc., and did not expose the marriage to unjustifiable dangers);
(3) provided that there existed a legitimate reason for avoiding conception.
So, what is a “legitimate reason?”
Pius XII used the phrase “
serious proportionate motives,” and said that he had spoken on periodic continence
"in order to put an end to the anxieties of conscience of many Christians," AAS, 50 (1958), 732-40, at 736." (ibid., p. 425-426). He called those motives “
indeed quite broad” (AAS 43 (1951), 855-60, at 859).
Consequently, the expressions ‘grave motive,’ ‘serious reasons,’ etc., are the equivalent of the expression “proportionate reasons.”
Fathers Ford and Kelly explain:
"The institution of marriage has as its primary, intrinsic end, from nature and from God, the procreation of children. Consequently, all those who enter this state are per se obligated to procreate, if they can. Consequently, they are obliged to use their marriage rights * during fertile periods often enough to discharge this obligation. But this obligation is an affirmative one, and individual couples may be excused from it temporarily or permanently for*
proportionate reasons…" (ibid., p. 409)
“…
couples who already have even one child cannot, in our opinion, be accused of a grave dereliction of their affirmative duty if they decide to have no more.” (p. 426)
“
…no couple can in practice be accused of mortal sin merely because of the use of [NFP] without sufficient reasons…” (ibid., pg. 450)
Similarly,
John Paul II taught:
The use of the infertile periods for conjugal union can be an abuse
if the couple, for
unworthy reasons, seeks in this way to avoid having children, thus lowering the number of births in their family below
the morally correct level. This morally correct level must be established by taking into account not only the good of one’s own family, and even the state of health and the means of the couple themselves, but also the good of the society to which they belong, of the Church, and even of all mankind. [John Paul II,
http://www.ewtn.com/library/PAPALDOC/jp2tb120.htm”]General Audience address, 5 September 1984]
See more here:
What is the morally correct level of children?
*