I can't cite sources, (maybe someone else can help), but I believe in earlier church history it was actually considered a form of sodomy.
It is certainly sexual in nature, and probably most people would categorise it as a form of sex, although whatever category man puts it in, it is God's view that counts and that we will be held accountable for. This is to say, that even if it is acceptable between husband and wife, between any other people it would be considered either adultery or fornication at best.