As time goes by I realize that one famous Psychologist wrote about feet and shoes,  Freud!  

Sigmund Freud viewed perversion like he did just about everything— as likely stemming from periods of disorder in childhood development..  Sigmund Freud claimed that people sexualize feet because they resemble penises.  Another  is that hormones and emotions drive people to imprint their arousal on to certain objects. These hormones and emotions allow them to respond to certain stimuli sexually.

So if the fit appears to look like a penis then women’s shoes and boots when viewed closely look like a vagina.  I have thought this for years.  I look at a set woman’s used sexy shoe or fashion boot and as I view the depths it translates into a vagina that in my view needs penetrating.  The combined odors of her feet and the leather come so very close to the odors of sex.  

My partner says it is masturbation, I thought hard on this and don't agree.

When I look closely and my head is buried in her shoe and I am kissing and passionately making out with the shoe it's so much more.  Then as I penetrate that other shoe I am not manipulating the shoe I climb on top and penetrate deep into the shoe.  In my mind this is full on sex and not masturbation.  Everything about it is the same as intercourse with a partner.  

Bob D New York