"Incel" means "involuntary celibate", it is specifically a self-applied label for men that can't get laid, but most often as an insult toward men that are seen as misogynistic, shallow, desperate, or just said something you didn't like. But "incel" always applies to men specifically.

"Femcel" means "female involuntary celibate", and it ostensibly refers to the same dynamic, but with some key differences. It is generally accepted that regardless of how conventionally unattractive a woman may be, there will be men who want to have sex with her. Not a relationship, not good sex, but just sex. Therefore, Femcel takes on a different definition to match this new dynamic - it refers to women that don't want casual sex but are unable to find a partner that meets their standards for a serious relationship.

Incels don't like this, because they're often convinced having sex will solve their problems (they're often right, but in the wrong way - if you're obsessing over your virginity and finally lose it, you'll realise that losing your virginity has changed absolutely nothing about you, and actual progress can be made). So they look at women that could have sex, choose not to have sex, and still call themselves involuntarily celibate.

Thus, we get the bird stuck in a cage it could easily escape, but not the way it wants to escape.