I can't give you opinions that are Christian based, since I am not a Christian, but here is my two cents on the subjects...
Marriage- I feel that it is a lifetime commitment to a woman, and if you're at that point with a lady, I think it's OK to get to the loving.
Pornography- If you come across it, you were probably looking for it. If you come across it by accident anyways, and keep looking, your natural desires and curiosities are acting up. I am no saint in this category, and have given more than my fair share of coinage to the industry. Always remember that most of what is portrayed in porn is FICTION. Almost all of the women that I have been with don't love being spanked, or liked facials, or most of the other BS that porn portrays as the kind of sex you need to be having to be considered a man. My best sex always came with woman whom I loved and was familiar with. The familiarity brings about the unified passion that makes sex truly great, and that only comes with a long term relationship. If you buy into the bullshit of porn, you will be disillusioned by the reality of the loving, and it will hinder your sexuality.
Masturbating- 99% of men admit to spanking it, the other 1% are lying. Just try not to break any records, and remember what I said about porn, keep it in the real world.
Checking out girls- If God doesn't want me to check out women, why did he make them so fucking hot. Always keep in mind that that fine piece of ass is someone's daughter/sister/niece/mother/aunt/etc. The moment you forget that she is a person, that is when I feel you are losing perspective.