Whatever one holds (no pun intended) regarding negiyah, it makes NO difference if the woman is pretty or plain, fat, thin, tall, short, blonde, brunette, or redhead. If something is assur, it is assur, not assur EXCEPT when the woman is unattractive.
Attractiveness is very subjective, and there are some very plain-looking people who are extremely attractive to SOMEONE. Just look at a lot of the shidduchim you see randomly. You might wonder what any of them saw in each other. That is the beauty of Hashem, who enables zivugim to find each other, even when others might not “get” the attraction. It is for that reason that we cannot rationalize that it is not oveir negiyah if the woman is not so pretty(maybe the McDonald’s cheesebruger wasn’t so treif, if I didn’t enjoy it). There are plenty of men who might not find a woman attractive, but still might have improprer thoughts if they came into physical contact with her (And maybe even if they did not actually come into contact, who is to say?).
Whatever we do, we make a conscious choice to follow or not to follow what the halacha is and cannot rationalize our actions away, but need to be hoenst with ourselves.