Unless you believe, as the SSPX does, but doesn't come right out and say explicitly, that a heretic can hold authority in the Church, be elected of God to have the force of Divine Law behind his decrees and teachings, that he can instituted ALL MANNER OF HERETICAL WORSHIP AND PRACTICES, just so long as he doesn't try to do it ex cathedra. But that is wrong.
Infallibility means that a pope cannot err AT ALL when teaching ex cathedra, but it is still a requirement that he be CATHOLIC in his public profession, that he not contradict the Catholic Faith in any way at all.
A heretic is not pope for goodness sakes!
Sure he is - look at John Paul II, Paul VI, etc. They are legit popes unless you contrive a variety of bizarre excuses to condemn them as non-Catholics. One day people will look back on this time and refer to these popes as proof that we must hold to tradition even in the face of a Pope who teaches with ambiguity.
Let us not forget the Cadaver Synod - a previous pope was dug up and put on trial. Who is not to say that one day the tombs of JPII, Paul VI, and the likes will not be smashed because of the horrors they allowed to befall the Church? They won't be declared anti-Popes because they certainly do hold the position of power, but they are definitely showing us how close to lies a Pope can get. Fortunately in the past we haven't had to deal with this - but now we do. The solution is not to declare them all anti-popes and run away and hide whilst spouting vitriol on the internet - the solution is to pray and wait whilst rejecting the lies.
Let us not forget the Cadaver Synod in which a legitimate Pope (Stephen VI) had another (Formosus) dug up, tried, and condemned.
This synod was then later reversed by ANOTHER Pope (Pope Theodore II) and Formosus restored and Stephen condemned. THEN - after all this, ANOTHER Pope (Pope Sergius III) reconciled Stephen (posthumously) to the Church.
One day, Vatican II and its popes will be condemned - just as Stephen was for his council which tried the previous pope. Sedevacantism is cowardice - it is men who are too afraid to deal with the real situation so they pretend it isn't happening.