Halakhah for Berakhot 39:6

And in any case, according to what appears from all that they, may their memory be blessed, have warned us, a man is not permitted to budge from their good teaching. And even if he finds himself a little devoid of desire, he should not say, "Since I find myself so, what do I care if I stare at women, since I know about myself that my impulse will not be aroused by this" - as many have said like this and stumbled. And about this, they, may their memory be blessed, hinted when they said (Sukkah 52a) that the impulse is very weak at first and continues to [grow until it] greatly overpower a man. And you, my son, be very careful about this and do not let your impulse promise you [that it will not overpower you], even if it gives you a thousand pledges. And that which you find a few stories in the Gemara that appear to contradict these words of mine - meaning to say that a few of them were not concerned about even that which our Rabbis, may their memory be blessed, forbade with women - this is not a contradiction to my words at all. As they were only a little lenient in the place of a commandment, as we found with Rabbi Yochanan who would sit at the gates of the [place of] immersion in order that the women would stare at him and give birth to sons as beautiful as he (Berakhot 20a), but he did not stare at them, God forbid; and Rebbi, and Rav Acha, who would take a bride on his shoulders for a commandment, as he did this in order to have her rejoice (Ketuvot 17a); and a few matters similar to this.