As always, I can only really speak intelligently about Catholic doctrine… but not for all Catholics (because I admit a lot don't really know what Catholics actually believe - they choose ignorance) and definitely not for all Christians/followers of the Bible.
I think “lust” as a sin - in terms of the Bible - means more than just the instant attraction of seing someone sexually desirable. Even to Catholics, this is a scientificly provable reaction (theory). Confidently, I can say that the proveable biological reaction to sexually appealing people is not a soul-damning offense. The sin comes later.
From “New Advent dot Org”, a highly respected member of the catholic knowledge base and the contents of New Advent are generally to be regarded as authentic catholic teaching:
The inordinate craving for, or indulgence of, the carnal pleasure which is experienced in the human organs of generation.
The wrongfulness of lust is reducible to this: that venereal satisfaction is sought for either outside wedlock or, at any rate, in a manner which is contrary to the laws that govern marital intercourse. Every such criminal indulgence is a mortal sin, provided of course, it be voluntary in itself and fully deliberate. This is the testimony of St. Paul in the Epistle to the Galatians, v. 19:
“Now the works of the flesh are manifest, which are fornication, uncleanness, immodesty, luxury, . . . Of the which I foretell you, as I have foretold to you, that they who do such things shall not obtain the kingdom of God.”
Full article on the matter found here: http://www.newadvent.org/cathen/09438a.htm
Mortal sin (the hell-damning-if-unrepented kind) to Catholics has criteria to actually be a mortal sin. The “sinner” must: 1) fully know it's a sin (intentional ignorance of Bible teaching could be a sin, however); 2) the sin must be commited voluntarily; and 3) it must actually be of a grave nature… that is to say that it is severe enough to intentionally (through act of the sin) alter the “sinner's” connection with God and His grace.
Example: Skipping Mass is a mortal sin to catholics. But, if the person skipping didn't know it was so, then doing so wouldn't actually be a mortal sin. Also, if the person knew, but was in a car accident (for example) that forced them to skip, then it too is not a mortal sin. Finally, if the sin is, let's say, a white lie that will help a person - like saying “that whipeout on the ski hill wasn't bad - no one noticed” when people DID notice and laughed and telling the person so might ruin them to skiing forever. It is a lie and therefore probably a sin, but it is not so severe that it qualifies as a mortal sin (and yes, there is distinction between sins in the Bible).
So, is “Lust” a sin? Well, yes. Is being subconsciously/instantly sexually attracted to a person a sin? No. It's uncontrollable… and definitely not a mortal one - the act was not voluntary. Going forward beyond the instant, biological reaction however IS a mortal sin… assuming you knew it was so in the first place (which if you are a Catholic/Christian and believe all this… then you probably knew it was).
For this and many more reasons, Catholic doctrine does not suggest and in fact discourages people from trying to judge sin and/or the state of a person's soul. That is left for God, as commanded by the Bible. Through His Word, we've gained insight, but ultimately it is too complex and divine to know for certain.
Finally… I think it's fine to see someone of beauty and think that they are so. Furthermore, I think it's fine for the thought to instinctually enter your mind. It is what you do with that thought after it enters that determines the nature of its potential inherent sin.
Long post complete. phew.