I’ve known several incels, and in my experience, their entitlement to sex is really just an extension of wanting to feel loved.
Men grow up in a near drought of love and physical affection. After about 5 years old, we can’t hug our best friends.
I think you really have hit the nail on the head about how sex is the only source for a lot of important emotions and needs that is socially acceptable in a toxic gender roles based system. Sex is the only place that physical intimacy, honesty, giving / receiving pleasure, tenderness, feeling sexy etc. can be achieved. Most people cannot express that correctly and can't see they can get it elsewhere. This causes tons of issues for people both within and without relationships.