Survival of the fittest is difficult to argue effectively or convincingly, as it is not us who are the architects of our own success. As “Anal Teflon” loosely pointed out, it is down to the success of our “ancestors” (by this I assume he means: people who most likely are no relation to him whatsoever, but from whom he is lucky enough to have inherited a fully industrialised country at the time of his birth). So we are lucky. That has been pretty well established; but does that fact give us the right to pull the old Survival of the fittest line? The Natural Selection, superior species ideal (nice to see relics of Nazi Germany still remain intact; the thesis of a racist bigot (by the name of Darwin), given a uniform and a gas chamber).
As for the reducing/eliminating sex argument; are you honestly saying that in their situation you would happily forgo what my well be one of life’s only pleasures? More than that, that they should be expected to forget/ignore beliefs relating to the sanctity of marriage? No. Should awareness be spread regarding the use of contraception? Yes. Is this a function of many charities? Yes. For most people this is really a no-brainer, but as you pointed out even a 10 year old could understand your argument. This is true, a ten year old probably could, however you should expect that a more intelligent and compassionate human being, hopefully like most members of this forum would find obvious flaws in your argument; as no doubt most have.