exposing oneself to proximate occasion of sin without necessity is sin in itself

http://www.archive.org/stream/alphonsusworks15liguuoft/alphonsusworks15liguuoft_djvu.txt (Preaching by St. A. Liguori)

Occasion of Sin

It would also be a miracle if we exposed ourselves to the occasion, and did not fall. According to St. Bernardine of Siena, it is a greater miracle not to fall in the occasion of sin, than to raise a dead man to life. St. Philip Neri used to say that in the warfare of the flesh, cowards that is, they who fly from occasions are always victorious. You say: I hope that God will assist me. But God says : He that loveth the danger shall perish in it. God does not assist those who, without necessity, expose themselves voluntarily to the occasion of sin.

Some also believe that it is only a venial sin to expose themselves to the proximate occasion of sin. The catechist must explain that those who do not abstain from voluntary proximate occasions of grievous sin are guilty of a mortal sin, even though they have the intention of not committing the bad act, to the danger of which they expose themselves. So, also, women must be taught it is a sin to take pleasure in being admired and desired by men, if this pleasure arises from vanity, and not from a wish to be married to them.

God does not assist those who, without necessity, expose themselves voluntarily to the occasion of sin. It is necessary to know that he who puts himself in the proximate occasion of sin is in the state of sin, though he should have no intention of committing the principal sin to which he exposes himself.

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By proximate occasions even saints have fallen, and persons on the point of expiring
have been lost.

We must carefully abstain from looking at persons, the sight of whom may tempt us to bad thoughts. St. Bernard says: "Through the eyes the arrows of impure love enter, and kill the soul." And the Holy Ghost says: Turn away thy face from a woman dressed up? Is it, then, a sin to look at a woman? Yes, it is at least a venial sin to look at young women; and when the looks are repeated, there is also danger of mortal sin. St. Francis de Sales says that to look at dangerous objects is bad, but to look a second time is still more injurious. One of the ancient philosophers blinded himself voluntarily in order to be freed from unchaste suggestions. It is not lawful for us Christians to destroy our sight physically, but we should destroy it morally by turning the eyes away from objects which may excite temptations.

St. Aloysius Gonzaga never looked at women; even speaking to his mother he kept his eyes cast down upon the ground. It is equally dangerous for women to look at young men.

It is necessary to avoid all bad company, and all assemblies where light bantering and flirting go on between men and women. 'With the holy thou wilt be holy . . . and with the perverse thou wilt be perverted.' If you keep company with the virtuous, you shall be virtuous; if you associate with the unchaste, you too will indulge in impurity. St. Thomas says that a man will be like the companions with whom he converses.