A”Z 17 on the bottom of Amud Alef.
Two points:
1. What you quote is the fact that one must not see inappropriate things because it will ignite his Yetzer. I merely added that there is a concept of someone knowing when something does not turn him on. Obviously, the Issurim mentioned by Chazal can not be transgressed with that as an excuse, because you are most probably fooling yourself.
The Chinuch adds that even those Amoraim, like Rav Yochanan and Reb Yanai and Reb Yehuda only did what they did when it was a Mitzva.
2. Even if something would ordinarily not effect you, by the mere fact that you are doing it even though you don’t have to, that draws you closer to Aveira. That is how I always learned the Sugya that you brought up. If there is no other way, there isn’t even a problem, not just Bedi’eved.
Either way, the fact remains the same, that if you know what you are holding by you don’t have to think that in a minute you will drop everything. When it comes to Arayos, you are probably just fooling yourself. So the concept of Al Taamin Be’atzmecha means you can’t know that you will remain a Tzaddik forever.