Jewish _male_ identity, obviously. "Supposed" is irrelevant (and highly debatable). What _is_ is that Jewish male identity views circumcision as one of its lynchpins. Kick away that support, and you create a powerful identity crisis.

Slippery slope argument says that "we shouldn't do X (even though X is inherently okay) because it might lead yet another set of actions – Y – which is a negative". I'm not saying that at all. Banning circumcision will lead to significant identity issues as a direct consequence, not as a result of some other intermediate effect that obtains as a result of the ban.