Yes, I believe it is a sin to attend the new mass for the following reasons.
1. The priest may not be validily ordained. Canon law does not allow attendance at a mass said by a doubtful priest.
2. The priest may not have proper intentions, depending on his probably poor seminary training. Canan law does not allow attendance at a doubtful mass, with a doubtful intention.
2a. Many "masses" are simply eucharistic "remembrances" or a "memorial" where transubstantation does not take place because the priest does not believe correctly.
3. Even if he's a priest and has a proper intention, so that the consecration is valid, this does NOT mean the mass itself (which is more than just the consecration) is valid.
3a. Many "masses" are just "eucharistic services" which do not offer God the sacrifice of Calvary, but only re-enact the Last Supper, which is not a complete mass.
4. The novus ordo's Offertory and Canon prayers are minimized and faulty. They lack the intentions as the True Mass, therefore the mass is not complete, which is a sin.
4a. The novus ordo's new theology of the mass is anti-catholic, anti-Council of Trent and sinful. The novus ordo is a protestant service with catholic "smells and bells".
5. The current liturgical law of the Church, Quo Primum, which Pope Benedict XVI said was still in force, does not allow catholics to attend any mass other than the 1962 liturgy/missal. So, all novus ordo masses are illegal and sinful.
6. The novus ordo liturugy is highly irreverent, sacrilegious and immoral. The liturgy does not promote prayer, silence and respect for God, therefore it is sinful.
6a. Examples: women altar girls, women eucharistic ministers, women in the sanctuary during mass, communion in the hand, priests making jokes, etc
6b. Example 2: Communion in the hand is an abominable sacrilege. One is not allowed to attend a mass where sacrilege takes place for such sins contaminate the entire ceremony.
7. The novus ordo's atmosphere promotes scandal and blasphemy by the laity's actions (i.e. improper dress, talking/laughing, dropping hosts on ground, etc). One is not allowed to attend masses/ceremonies where the atmosphere is blasphemous.
**All of the above is based on objective facts and Church law. This does not mean I judge any novus ordo catholic laymen who is trying to figure out their path, amidst the choas of our day. I do not judge ANYONE guilty of mortal sin, I'm only giving out the reasons why these actions are sinful. God alone will judge the hearts of men and He will lessen the guilt of these sins for different people, based on many different factors. It's just our job as Catholics to educate people so that the offenses and injustices to God and His Mass are stopped.**
Required reading:1. Fr James Wathen - "The Great Sacrilege"http://fatherwathen.com/product/the-great-sacrilege/Required Listening**See the above site for free audio sermons you can listen to about the dangers of the new mass and the new catechism. (see below for sermons on our present topic)
http://fatherwathen.com/product/19770417-why-catholics-may-not-go-to-the-new-mass/http://fatherwathen.com/product/19960806lack-of-reverence-in-the-conciliar-church/http://fatherwathen.com/product/19821003-the-mass-through-history/http://fatherwathen.com/product/19770710-the-brainwashing-of-modern-catholics/http://fatherwathen.com/product/19830710-the-insidious-heresy-of-liberal-catholicism/2. Cardinal Ottaviani's "Intervention"http://www.catholictradition.org/Eucharist/ottaviani.htm