It is "natural" when someone offends you, that you want satisfaction - nature seems to favour, with time, equilibrium.
It is natural to hunger and thirst. Indeed, one could even argue that gluttony is "natural" - in a dog-eat-dog world, it would seem to be good, from a survival standpoint, to ply one's self with as much rich food as one can, while the getting is good.
It would even seem natural to lust - the "need to breed" guarantees us that purely natural form of immortality; it is the drive which causes animals to breed, and arouses our interest in the opposite sex. Of course, human "cleverness" (matched with laziness) has taught us all sorts of ways of catching the "buzz" off of this impulse, without the normal consequences (much like stuffing one's self, but then throwing up for vanity's sake.)
There is a certain value in the western "natural law" tradition...however, where to draw that line between "natural" and supposedly "unnatural" is difficult, since much of what is undesirable (or at best a comprimise) is deeply imbedded into the natural world as we currently experience it.
The safest thing to say, is that this world as we now experience it, is not what God intended for us; it's something our first father chose, and now it is what we're born into. It's filled with all sorts of unpleasantness and blemishes... humilitations and embarassing weaknesses (particularly when one considers the preciousness of the human soul, how it is angelic, able to receive the grace of the Holy Spirit.)
There is a logic in killing wantonly, might makes right, etc. However, we have been privileged in these last days with some good news, that there is more than this fallen world; not only is there a return to Paradise, but the promise of things much greater than even Adam knew (though he would have come to know, had he persevered.)
The Christian life is not legalistic righteousness; it is the acquisition of the "righteousness" of God, of something totally above the natural order (yet, due to the Incarnation, very much within it). God is totally unlike His creation (which is why He can never be known in His "essence.") However, we do know God from His "energies" (via the Incarnation of God the Son, as the God-Man Jesus Christ), and truly awesome is the possibility of being assimilated by this grace (thus gaining the qualities of God...theosis aka. "divinization").
Thus, to become "like God" is to become like He Who is above and unlike this world. It is to tread into immortality.
The immortal, spiritual man will see things in a totally different way than the one labouring under the heartfelt conviction that this world is "all that there is" (not simply in terms of there being or not being an afterlife, but also in terms of it being our only "way" here and now). He will no longer fear death, nor be consumed by avarice (for the things that this world offers, or even things that are perceived as "needful" for earthly survival, but contradict evangelical principles.) He will suffer for truth (where as naturalistic survival would demand comprimise, unless one were physically more powerful than his foes), and with his Master will "overcome the world."
With all of this in mind, one can perhaps be better able to touch upon the issue of sexuality.
The Fathers maintain that the current mode of procreation is not what would have been encountered in "unfallen" man. Does this make human sexuality 'bad'? No, no more so than mortality. Either can be foul and tragic; however both can also be met with dignity and grace. Just as the Christian who loves God has nothing to lose from his death (though it surely speaks to his being born of fallen stock), neither should he be ashamed if he approaches human sexuality with the same Godliness.
Seraphim
