"What is so wrong with sex within marriage that makes God require priests to forgo it?"
There is absolutely nothing wrong with sex within marriage - it is good and we should have as much of it and as many children as possible.
If it was wrong, the Church would require all of us to forego it. The grace of the celibate calling is that the celibate offers up in sacrifice something that is good to God - his or her fertility. If it was not a good that they were sacrificing for God, then their sacrifice would have no meaning and no value.
They offer something that is very good and precious to God in order that they may achieve a higher good - the total consecration of self to God, above and before all others.
If people approach celibacy with the idea that sex is something bad or grubby they miss the point. Also those who approach it as a negative deprivation of something miss the point, and in both cases they will struggle with it. It should be positively offered to God as a sacrifice of a very great and holy gift.
To tell you the gospel truth, this is the thing that causes me to hold priests most in awe. I mean, I love God, but to give up sex for life?
I could take a vow like that, in a moment of headstrong belief in my own power. But it'd be a vow I would be destined to break.
Soon.
I do not know how they do it. Truly I don't.
And I am pollyannaish to actually believe that the priests I meet really are celibate (even if you installed hidden cameras all over their house), and that they really have that level of committment.
It isn't faith. I have faith. I'm sure God is there, as sure as any priest. Had visions and the like. But even with certitude and genuine love for God, there...is...just...no...way...I...could...do...it.
Given that I am a pollyanna and believe that almost all priests really DO do it, really do keep their vow of chastity, I see each and every one of them as living proof that men truly are sometimes CALLED by God to serve in that capacity. Without that grace, that charism of the holy spirit to protect them against the screaming hormones that sometimes flow in my veins, they could not do it.
The fact that priests take that vow, and are celibate, is the strongest of all proofs to my simple eyes that they really are under a special grace of God extended specifically to be able to do the priestly role.
I DON'T think this necessarily makes them HOLIER (although it ain't hard to be holier than me), but I DO think that it demonstrates that one does not CHOOSE to be a minister of God. Rather, God chooses you, and makes you capable of fulfilling that role, and the greatest sign of that is that He makes you able to fulfill a vow of celibacy.
Precisely because I could never do it, but priests routinely do, I hold celibacy as a sign of divine grace, and the most imposing way I could possibly imagine whereby God could really demonstrate to the world who HE wants as his priests.
That said, I've always found it funny to call them "Father", given Jesus' comments about not calling anybody "father". Now, I assume Jesus didn't mean "Don't call your dad you dad", but I figure he was referring to customs of the time of doing what we Catholics do with our priests, revering them by calling them "father".
Of course, I don't know what else I'd call a priest if we didn't call them father..."your blessedness", perhaps?
Anyway that was all stuff that didn't need to be said, but now I can't take it back, can I?
But I think it has been perverted and many in the Church view sex, and by extension women, as something evil.
The historical context could go either with celibacy or with married clergy. Has tradition created the celibate priesthood and therefore the sacrifice you describe?