I'm afraid, spiritus, that Myrna is correct on this matter. Though many people who attend the SSPX are not "dogmatic anti-sedevacantists", many are. Bishop Fellay is. I was at the October 2010 Angelus Press Conference where Bishop Fellay made it quite clear that he believed Benedict XVI and most of the Vatican cardinals are heretical members of the Catholic Church while sedevacantists are schismatics who are outside the Catholic Church. It was listening to his conference that convinced me that he was not going to "accept a deal" from the Vatican anytime soon, but he also despised any argument made by sedevacantists and wouldn't mind if they were all expelled from SSPX chapels.
The bishop made an airplane analogy for the Church. He said that the Church is like an airplane. Catholics are in the plane being flown by the pilot, the pope. The Protestants won't get on the plane while sedevacantists and Orthodox are flying in a plane without any pilot. He never connected, however, that he is refusing to get on the plane with Benedict as the pilot because it is flying in the wrong direction. He had noted earlier in the conference that Benedict was making some right moves, but he was essentially still following the path of Vatican II, i.e., he was flying his plane in the wrong direction.
Many anti-sedevantist tradtional Catholics, such as Bishop Fellay, neatly compartmentalize the issues so tightly that they can rail against both a pope who is leading his flock astray (and even claiming that he is a ravenous wolf at times) while also condemning sedevacantists who won't follow that shepherd into the pit (or by eaten by him).
But spiritus is correct that this attitude is not on display everywhere on CathInfo though any honest reading of this forum will show that there are, at least, a few "dogmatic anti-sedevacantists" on the forum. Even these, however, seldom raise their heads except in off-handed ways.
The fact that sedevacantism can be rationally discussed on CathInfo, even if few minds are actually changed, is a blessing, I think. The other traditional Catholic Forum to which I belong does not allow the sedevacantist thesis to even be discussed and the programming automatically changes any derivation of the word "sedevacantist" to "BLEEP!". I think it is childish and since questions and comments often lead directly to the sedevacantist theories (e.g., "What are we to think when the pope...?"), the whole forum grinds to a halt. The forum owner is a parishioner of an SSPX parish and has made it clear that he believes sedevacantism itself is anti-SSPX and he will not allow condemnation of the SSPX.
Legally, one must say that today's sedevacantism is a thesis, a theological probability. This is because there is absolutely no authority that can legally bind all men's consciences on the matter. On the other hand, the sedevacantist theory is either true or not whether it can bind one's conscience or not. As much as one believes he can fly when he jumps off a tall building, he will fall to the ground. Of course, this example can be used to demonstrate either side. The fact is, that whether we have a pope or not is a fact that will continue to be a fact no matter what any of us believe. But it is not a "dogmatic fact" as is, say, the Immaculate Conception.
One can choose to believe either side of this debate and still be fully incorporated in the Catholic Church. Outside the Dimond Brothers and Saint Gertrude the Great, the number of sedevacantists who hold that anyone who disagrees with them are outside the Church for that reason alone is exceedingly small. On the other hand, it seems that the vast majority of people who think that if one does not fully accept the claims of the conciliar popes to the papacy then one is, by that reason alone, outside the Church. This is especially true if you include those who think "ordinary form" and "extraordinary form" are valid expressions of the "one Roman Rite". And if you consider those who believe that virtually all "humankind" is somehow incorporated in the "Church of Christ" (even if they despise Christ outright), the only people whom they consider to be truely damned are sedevacantists and other traditional Catholics.
Thankfully, thiese latter groups are not too vocal on CathInfo.