First of all, I would recommend you have a little more tact on how you describe your sensual arousal. If you do not know what this means, please do not post here; go talk with your priest.
Next, if a young man sees a beautiful woman/girl, there is nothing wrong with this. If he is sensually aroused, however, he should definitely be careful, as it is quite common and natural for him to next have sensual images conjured up in his mind's eye (ie: he will start thinking some sort of sexual (adulterous) thought).
Remember, Our Lord, in perfecting the old law, said: "But I say unto you, that whosoever looketh on a woman to lust after her, hath already committed adultery with her in his heart." Fr. Haydock (from the Haydock version of the Douay-Rheims bible) said in his commentary on these verses, "Jesus Christ here perfects the old law, which makes no mention of the acts of the mind and will. (Menochius) ... Whatever is an immediate occasion of sin, however near or dear it may be, must be abandoned (Menochius), though it prove as dear to us, or as necessary as a hand, or an eye, and without delay or demur. (Haydock)"
So, then, you should be careful on how you look and think of women and girls. Its perfectly natural to be attraced to them, but be careful in letting your mind wander into what is against Our Lord's law, okay?