Involuntarily celibate? The term is I’ll-conceived. They are not choosing to characterize themselves by reference to their marital status (single), but rather by reference to the fact that they are not engaging in the marital act. Single men who engage in the marital act with women are referred to as fornicators, with other sexual acts with women being referred to as sodomy, or without women being referred to as self-abuse. Chances are, all or nearly all so-called “InCels” are actively engaging in self-abuse. The first word of advice would then certainly be, absolutely, definitely, “voluntarily”, cease abusing yourself.
Who says?
The term perfectly expresses what they want to express (though not what you want to express, apparently).
Single men who engage in the marital act with women are referred to by one guy in new jersey as fornicators [...]
Again: You are free to refer to them however you wish. But they are also free to refer to themselves however they wish.
The terms which you wish to apply ("fornicators," "sodomists," etc.) may be relevant and useful for the kinds of conversations you want to conduct - but are not pertinent to the discussion they want to conduct.
Regards,