An obsessive sexual interest in a non-genital body part, such as the foot, or in a non-human substance, such as latex.
Periodically, analists claim that Frot is a fetish.
It's not.
Frot, like penile-vaginal intercourse, is genital-genital sex.
Only if one is willing to assert that the way human beings are conceived is a fetish, can one also assert that Frot is a fetish.
Anal penetration, on the other hand, fulfills the definition of a fetish.
"Men into anal" display an obsessive interest in not one but two non-genital body parts: the anus and the rectum.
No matter how you cut it, that's a fetish, and an odd one, for it defines two of the body's organs of fecal excretion as the ultimate objects of sexual desire.
So: putting your penis in a hole full of fecal material is a fetish.
Treating your anus like a vagina is a fetish.
Engaging in penile-penile sex -- Frot -- is not.