Well, if you regard the NO as a non-Catholic rite, then it would be the same as going to a Methodist church. I, obviously, don't buy this, but that seems like a logical conclusion for someone who does.
Conversely, if you do think it is a Catholic rite, even an imperfect one, It is hard for me to see how you could regard it as a sin.
Soulguard, aren't you a sede? If so, you might find an SSPX parish more concerned about that than the fact that you have attended the NO.