YES, IT IS (see I can do all caps too

)
As you can see from Lumen Gentium (a DOGMATIC document from Vatican II)
Contraception is a matter of morals, taught by the Magisterium in the name of Christ.
Canon Law must be read in light of the other documents of the Church. It is a black and white issue. It is a heresy to deny a faith or morals teaching that has been taught definitively by the Church, even if it has not been declared ex cathedra.
This forum has officially lost all credibility for me.
Your citation from LG actually backs up my point. “religious assent” and “religious submission of the mind and will” are NOT the same thing as “divine and Catholic faith”. Not by a long shot. The Church makes these distinctions for very important reasons.
The things that we are required to believe with “divine and Catholic faith” are not that numerous. A great summary of these things are found in, but not limited to, those statements in the Nicene Creed. For example, “we believe in one God” “we believe in one Lord Jesus Christ, the only Son of God”, “he was crucified, died and was buried”, “on the third day He rose again”. THESE are thing I must believe to the very core of my soul, or as the Church puts it, with “divine and Catholic faith”. If I am obstinate in not believing these things, I am a heretic.
"Heresy is the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith, or it is likewise an obstinate doubt concerning the same;
As you can see, ONLY the obstinate denial of the truth of matters such as those referenced above can rise to the level of heresy at all. Not contraception, not the rule about eating meat on Fridays.
Things such as the teaching on contraception require, as you so nicely pointed out from LG, “religious submission of the will and intellect”. The threshold is set much lower. In other words, I am not required to believe to the very core of my being that contraception is wrong. I am only required to submit to this teaching in practice, and to admit to myself that even if I do not believe in this teaching, the Church MAY still be right about it and I may be wrong or misunderstanding the teaching. That is all. Now, if I do not follow this teaching in practice, it is (according to the Church) a grave sin, but it is NOT , nor will it ever be “heresy”.
Let’s have an example:
If I do not believe in my heart and soul (a.k.a. with “divine and Catholic faith”) that Jesus Christ is the son of God, can I receive communion? Of course not. Even though I have not “committed” any grave sin, just not believing that Jesus is actually the son of God places me outside the Church, for such a belief is a heresy.
Now, if I do not believe in my heart and soul (with “divine and Catholic faith”) that contraception is wrong, but I do my best to follow the teaching in practice (I do not use contraception) and I am willing to allow that the Church may be right and that I may be misunderstanding the teaching (a.k.a. I give the Church a religious submission of will and intellect on the matter) can I receive communion? YES! I am not a heretic even though I do not believe this teaching with “divine and Catholic faith” because quite simply, I am not required to. While practicing contraception and shouting from the rooftops that the Church teaching on contraception is a load of baloney may pit me in a state of mortal sin and unable to receive communion as a result of said sin (according to the Church), it does NOT make me a “heretic” because the level of faith required for this teaching is simply not there.
Will someone who knows about this PLEASE back me up on this? It is the absolute truth.
Do not take my word for this! Ask any knowledgeable priest or canon lawyer or bishop or even BENEDICT himself.