What is so wrong with sex within marriage that makes God require priests to forgo it? It seems the Church has a historical precedent to have both married and celibate priests.
I don't think it matters is a priest would be hetero or homosexual if he is celibate. Unmarried, he shouldn't be doing it with anyone!
God doesn't require it.
Those who insist that God requires it make God look very confused:
He calls married men to be priests in the Eastern Rite, but only single men in the Latin Rite.
Oh, and He calls some Protestant married men to the priesthood in the Latin Rite after they convert, but He doesn't call any Catholic married men to the priesthood in the Latin Rite.
Mandatory celibacy is a Church discipline and can be changed at any time.
He doesn't. According to SCRIPTURE, sex is REQUIRED for married couples. "Do not deny each other...", etc. (1st Corinth 7.) No exception is made for priests.
"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.