This fallacy states that what is true of the part is true of the whole. What comes to mind here is the perception of Involuntary celibate, which means to not be in a sexual relationship when one is desired. Originally, the term was coined by a woman to describe herself, and the original community was all genders and supportive.
Of course, over time it’s morphed to become the incel movement, which is comprised of bitter men who want to attack women. Now obviously there are many people who have the status of being involuntary celibate and are not like this, it’s a fallacy, but there is no positive identity they can adopt and so no rational, positive (logical) discussion possible.
In a way it can be a type of stereotyping. For example, thinking of Adult Virgins or Forever Alone. There can be many reasons for this identity, such as abuse, disability and so on but the general stereotype (composition fallacy) is that everyone in these groups is socially awkward, which has the effect of just discounting the whole group.
It also projects outwards. When I look at various isolated communities online I notice the discussions are focusing on people with very high standards in dating for example, and in a way it’s also a composition fallacy because if you wanted to find contrary evidence of people with reasonable standards, it would be possible (but perhaps not easy!).
[Note: the “division fallacy” which is essentially a reversal of the above, the whole is the truth, and so it applies to the individual parts also.]