I too am a foot fetishist, but I confronted this issue from a different perspective which I found enlightening.
I dated a woman who was a "foot fetish model" at Footworship Palace in New York (known as FWP). Clients (all of them men) paid to sniff, lick and worship her feet while they jerked off. Nothing else is allowed at FWP and she didn’t do anything else.
As we grew closer, her job started to bother me but I didn’t complain and I didn’t pressure her to quit because I knew she couldn’t find other work paying nearly as much and would end up resenting me if she quit on my account. However, I didn’t consider her my exclusive girlfriend and I made that clear.
Our problems arose when she wanted me to become monogamous while she continued working at FWP. In her view, what she was doing at FWP wasn’t sex so she was being monogamous to me all along.
I ruminated on her request and then I rejected it.
I concluded that analyzing foot fetishism by describing what physically occurs in a clinical, objective sense is a non-starter. To the foot fetishist, sniffing and/or licking and/or whatever that fetishist digs doing with feet, is sex - plain and simple. If one orgasms from doing something, then to that person that something qualifies as sex.
I posed an analogous (and much more innocent) hypothetical scenario in which my job was to sit in a room with a woman with the door closed and merely talk to the woman - never making physical contact - as the woman masturbated to my words. I asked her if she would consider my hypothetical job consistent with monogamy. She admitted that she would have problems with it. End of story.
I am not trying to moralize but only to pierce through what I consider - with all due respect - your weak rationalization. Do it if you wish, but call it what it is.