Someone who's family sometimes has a certain modest young couple over their house on Shabbos for a meal.His Yetzer Harah can get him excited if the wife is a pretty girl. Would anyone have any good advice for someone in this situation ?
Below is part of an answer I posted with respect to another question, but it is directly applicable to your question:
At [Yoma 29a], the Gemara quotes Rav Nachman who says that sinful thoughts are worse than sinful acts. Rabbi Adin Steinsaltz, in his [Daf Yomi notes], comments that here the Gemara is using the term aveira and when it does, it is usually talking about sexual temptations. So it appears that Rabbi Nachman is saying that sexual thoughts and fantasies are worse than actual sexual acts. Rabbi Steinsaltz then cites Rashi's explanation that this does not refer to the severity of the sin, but to the lust that accompanies thinking about the sin, which is even greater than what exists during the sinful act itself. However, Rabbi Steinsaltz notes that most other commentators understand the statement to be referring to the severity of the thought and the act. E.g., in the Moreh Nevuchim, the Rambam explains that the mind, the intellect, is on a much higher level than physical activities. Therefore, sinning in one’s thoughts creates greater damage to the person than does an act of sinning. I like the Ohr ha-Chaim's approach which says that once someone has sinned physically, he has satisfied his inner need and is ready to begin a process of teshuvah – repentance – leading to atonement. However, sinful thoughts which are never acted upon, never satisfy the person, and he will never try to undo or repent from them.
So from the point of view of the Sages, your feelings of arousal at the sight of pretty women is natural, but also more sinful because you might cause yourself to do an aveira, whether it be engaging a prostitute or masturbation. The usual answer I hear is that if this is a problem for you, you need to find a wife ASAP so that you have a permissible outlet for your sexual urges.