Homosexuality is said to be against nature because it goes against the natural order of a man with a woman. Christian conservatives point to Paul stating it in Romans 1:26:
"For this cause God gave them up unto vile affections: for even their women did change the natural use into that which is against nature.*"
But celibacy goes against the nature of heterosexual human desire given to us by God, undermines the traditional family unit and if everyone became celibate (like the argument is with homosexuality) the world would die out. Celibacy goes against the man/woman example in the Garden of Eden (like homosexuality) and goes against God's command to; "Be fruitful and multiply." Yet Jewish tradition says Moses chose to be celibate after his encounter with God. John the Baptist was also celibate. Paul said to prefer celibacy to marriage like himself and Christ lived that example as did the Disciples who weren't already married prior to meeting Jesus according to Tertullian. The early church fathers advocated celibacy over marriage. Celibacy is the ideal way of existing according to the New Testament because Paul said the attention you would give your spouse, would instead go to God, but celibacy is just as much against nature in Romans 1:26 as homosexuality is if the natural order is a man being with a woman for procreation. So where is this emphases on marriage and the "family" Jesus Himself downplays (Matthew 12:46-50)? The Bible says there is no marriage or procreation in Heaven and this shows the lack of importance Christ gave to this now and in the next world.
*I don't believe it's talking about homosexuality, but I'm using this as an example of the unimportance of heterosexuality over homosexuality in what Paul is saying.