I find it ridiculous that one would be in hell for eating meat on a Friday and another person wouldn't, just because they were born 50 years apart. It's either a sin or it isn't.
Well, maybe if we nuance it a bit:
Most of the Church's prohibitions come from an understanding of the Natural Law. The Natural Law is a kind of standard that says there are some actions that are simply incompatible with moral nature - or, there are principles that are built into creation and the human person that would be an offence against nature to break; hence, it is wrong to attack the weaker because it is a violation of a fundamental moral principle that is built into the nature of humanity. Some of the Church's prohibitions in this regard are: murder, polygamy, usury, exacting revenge, calumny.
Other of the Church's prohibitions are contingent on knowledge. It is not known at this time when "ensoulment" occures. Ensoulment is the time at which the soul is united to the body, and hence a human person exists. Because this is unknown, but we have reason to believe that it occurs very early on (certainly before birth, which, other than a change in environment, is really a rather arbitrary distinction as regards personhood), the Church condemns all abortion at all stages of development. But, if somehow it was able to be demonstrated that ensoulment happens at a time later than conception, the Church would revise her prohibition accordingly.
Finally, some of the Church's prohibitions (and requirements) are based on commands that are directed toward some good for the people under her charge. It is not an offense against nature to eat meat at any time (at least, unless it's part of some elaborate and unlikely suicide attempt). But still, the Church requires abstinence from meat on Fridays as a way of entering into the mystery of Christ's suffering. It is still required to abstain, unless one does some other sacrifice or remembrance on Friday (such as a good work, or some kind of mortification). The sin here is in disobeying the Church's command which is binding on all the people under her charge. After all, we all understand authority, and if a teacher orders a student to do something, and the student refuses to do it, we say that the student is in the wrong. So, the question is of course whether the Church has authority over her members; the Catholic philosophy is that she does.
Itaque, yes, somethings are either a sin or they are not (violations of natural law). Other things are sins secondarily (the sin is in disobedience to legitimate authority).
