One comment that I hear often during Lent is, "God won't send me to Hell because I ate meat."
The person who makes that statement is usually shocked when I respond by saying, "You know, I agree. You are right".
That usually gets their attention long enough, then catechesis begins.
First: God doesn't send anyone to Hell. We get their on our own.
Second: Eating meat on a Friday during Lent is not a sin, but the attitude of a Catholic who chooses to is.
Abstaining from meat is not a doctrine or a dogma of the Church. It is a practice much like priestly celibacy. The Church has chosen to enforce this practice because of the great benefit which it gives to individual Catholics and the Church as a whole. The Church has had 2,000 years to develop these practices. It doesn't create them on whims, serious thought and prayer goes into them.
So, where does sin come in? Simply put, the person sins by eating meat on a Friday during Lent because the individual presumes to know better than the Church. The Church has spent 2,000 years thinking about these topics. How much time have you spent? I thought so.
So, is it a mortal or a venial sin? Well, that depends on the attitude of the carnivore.
Basically what I want to say in this post is that it's not about meat at all. It's about denying ourselves, uniting with other Catholics in a common practice, and being obedient to Church law. There is so little that the Catholic Church requires of her members, not eating meat on 8 days isn't that much of a sacrifice. Just remember, you could be ordering a Ramadan meal or keeping Kosher all year long.
On a related note: Remember the Meat Police?