I attempted to answer that in #72. Hermann gave a very detailed answer in #75. The differences I have with Hermann are probably differences in emphasis or wording, at most -- differences in corner cases not likely to present themselves to real people often. The universal answer is, when in doubt, talk to a priest, and if you feel the priest is one of those liberal pat-on-the-back servers at the Amchurch Cafeteria, find another priest. In that sense it is not up to individual conscience but rather to an honestly examined conscience. For example, if a parent does not work on his parenting skills, and then says, -- I am not a very good father, let's not have more children -- is not using a just reason, but I suppose circumstances exist when inability to properly parent children is a just reason.
Another distinction is between an obligation and virtue. I made some posts even here, where I did not carefully distinguish between the two. To love your spouse and to love God is an obligation. This is why artificial birth control, or generally sex excluding a possibility of procreation is a moral impossibility. To give of yourself in order to put forth more fruit, -- for example, have another child, when the effort is legitimate (no welfare, no artificial insemination) but extraordinary, is virtuous while not obligatory.
Self-examination is also needed to distinguish between ordered and disordered sexual drive. A strong sexual urge during fertile time might be a call from God to have another child. But sometimes we have sexual urges that are disordered, this includes sexual urges that are badly timed; it is then an obligatory thing to suppress them. Suppression of the sexual urge in order to fulfill a commandment of God, such as vows of celibacy or monogamy, or abstinence due to fasting are in that category, but abstinence due to inability to support another child is there as well.
Generally speaking, of course, the sexual urge is a signal from God that He wants us to procreate.
I appreciate your reasoned and thoughtful response. And that you take the time to express it fully.
As to sexual urges--can they not also be there to further the connection between spouses, not just as a signal to procreate? And I guess oral sex is out of the question, too? (Don't mean to be graphic, sorry, but I think it's a part of this whole discussion.)