Saying "Christianity believes X" is likely to be not true for some denominations for any X other than "Jesus is the son of God"
Even if you restrict yourself to Protestantism in the US, you will have trouble generalizing too much; the doctrine of Sola Scriptura[1] is probably the only other universal there.
With regards to sexuality, there is very broad agreement that extramarital sex is sinful, but not much more agreement beyond that. Most protestants are accepting of contraception within marriage; Roman Catholic doctrine is opposed to any use of contraception (to the point of the official stance being that HIV positive married persons should never have sex rather than use a condom), however somewhere between "more than half" and 90% of western Catholics disagree with this.
James Dobson (A well known Evangelical Protestant psychologist) gets plenty of angry letters any time his stance that masturbation is a normal and healthy part of developing sexuality comes up.
They see sex as equally procreative and unitive -- so if you explicitly block either aspect, you're misusing it. (Catholic perspective, anyway, and probably significantly over-simplified).
Not sure why child post from aidenn0 is flagged dead, the way I read it it was correct and unoffensive but for what I know it might have been changed after flagging or I might have misread it.
