I am not very educated in the doctrines and such, but am married and feel like replying. 

I think that the spiritual violation of this is that basically God commanded us to multiply...fill the earth with lots of babies to raise to love and serve him. 

I’m not currently aware of any prohibitions on sterile/infertile couples against the marital act, as it fosters intimacy and prevention of a multi syllable word, starts with c (can’t recall right now) but is essentially sex or intimacy outside of marriage. 

Again, I’m a new Catholic, so do not quote me. If I am wrong let the shredding of myself begin. 

I've always held the understanding that sex serves the purpose of giving humans an outlet to express intimacy (obviously within marriage because being intimate with a variety of people is an obvious way to drive yourself insane) and this intimacy gives rise to children thus continuing our existence as a species. But when women go beyond child bearing years or one of the partners is sterile they can still have sex as an intimacy outlet.

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This is essentially correct.  Marriage and the marital act have multiple ends.  Secondary ends of each include mutual bonding and the satisfaction of concupiscence.  As Casti Conubii states, these ends are lawful to pursue so long as they are pursued naturally (i.e., without contraceptive intervention), and therefore duly ordered to the primary end of procreation (Denz. 2241).
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There are a variety of different sources one can use to prove the point. The Code of Canon Law enumerates a many different impediments to marriage, but does not regard marriage between sterile persons as invalid or unlawful (marriage between impotent persons is, however, invalid; this is because in such an arrangement there is an actual inability to copulate, not merely a natural defect to conceive).  Casti Conubii also describes such relations as lawful (see Denzinger citation above).  Theologians since the 1850s have occupied themselves with questions of marital relations during sterile periods ever since periodic continence became known to man, and they have all affirmed its morality, including multiple decisions of the Holy Office dating back to before Vatican I.  Prior to the 1850s or so it's more difficult to find detailed theological treatises on the morality of sterile relations simply because the biology behind conception was not yet particularly well understood, but since then there is a plethora of material.  You might look up the work of Griese, Wayne, and especially Vermeersch (who drafted Pope Pius XI's Casti Conubii) if you'd like even more information.

You well, that’s actually a good question... I’ve heard that if a man is impotent that marriage  won’t be allowed because it can’t be consummated. People who are sterile shouldn’t be finding out they are sterile until after they are married I suppose. (Unless they had some health issue that lead to its discovery) So, I’m assuming marriage is allowed. Plus, miracles happen all the time!

Impotency and sterility are not the same thing.  Impotency is the physical inability to have marital relations.  Sterility is the inability to have children.

Impotency is an impediment is marriage, and a moral man would know  if he impotent prior to marriage.

Sterility is not an impediment to marriage (a man or a woman could be sterile), and, absent some health issue, a moral person generally wouldn’t know of their sterility prior to marriage.

I did use both words, and explained them as such. You said exactly what I did, but just felt like correcting me.

It would appear you need to grow a thicker skin. I didn't interpret his post as hostile towards you at all, as I read this entire thread.
I interpreted his post as clarifying for third party readers, since both terms were thrown out there.
We don't believe in micro-aggressions around here. We believe in having skin thicker than 1 cell.

Mith has the right answer from Pius XI's Encyclical Casti Conubii.

Pope Pius XI teaches that there's the primary end of marital relations (procreation) and the secondary (fostering mutual affection, allaying of concupiscence).  He explicitly states that if the primary end is not possible (e.g. infertility), marital relations may be had for the secondary ends.  The principle is that the primary end can never be positively or willfully excluded while seeking the secondary ends, i.e. that the primary can never be made subordinate to the secondary (which is why IMO NFP is wrong, based on this teaching).

Under ordinary circuмstances the primary end shouldn't be subjugated. But in extraordinary circuмstances, with serious reasons, couples may practice periodical abstinence

False.  There's no exclusion in Pius XI's teaching for "extraordinary circuмstances".  The burden is to explain with any particular practice how the primary end is not subordinated to the secondary.  That has never been done for NFP.  Closest anyone comes is to claim that this isn't the case if someone is "open to life" (code language for ... would not have an abortion if NFP failed).  But one could say the same thing of someone who would not have an abortion after a child is conceived because of a faulty condom.

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"Openness to life" is a post-conciliar concept introduced by Paul VI.  Right or wrong, it's not something that has ever been commonly stipulated by the pre-conciliar theologians or popes who affirmed the morality of periodic continence.
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The poster in question had the right conclusion but the wrong explanation, or at least a vague one.  What does "subjugate" mean?  In Casti Conubii, Pius XI taught that no reason excuses from the negative precept to not deliberately frustrate the marital act.  This is in reference to contraceptive behavior, as is made clear by his simultaneous affirmation of the intrinsic morality of sterile relations precisely because they entail a due ordering of ends.  At any rate, whatever the poster meant by "subjugate" it would not be correct to say that Pius XI made any exceptions to the necessary ordering of ends; he affirmed the morality of sterile relations (and periodic continence directly and specifically shortly after CC's publication) precisely because in such relations there was no intrinsic disordering.  It's not an exception, it's a completely different thing.
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The relevance of grave necessity is that grave necessity can excuse from affirmative precepts, like the precept to go out and multiply.  A separate but contributive point to the morality of periodic continence.

NFP is a piece of junk, "we must perform on this day of the month", takes all the fun out of it. More importantly the woman naturally has no interest during those days. At best it is an avenue of relief for the man. Practicing NFP is a sacrifice in and of itself. If their is danger of death to the woman, and the NFP method is used as an "escape valve", from total abstinence, maybe a 6 times a year, it likely is not a very serious mortal sin? Maybe a venial sin?  

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The popes and theologians who taught periodic continence (which is really its proper name, not NFP), Pope Pius XII most notably, were careful to point out that the mere presence of a sufficient reason is not enough to make its practice allowable; the motives of the couple must also be good (though if they are not, the unlawful use of periodic continence doesn't "become" contraception; it's a different type of sin, a sin against marriage as opposed to nature).  The Novus Ordo practice of imposing NFP as though it's something that should indiscriminately be used flies in pretty stark contrast against what the pre-conciliar authorities taught.