Can my spouse drop me from their insurance if we are not divorced?

Yes.  Sometimes this is done for entirely legitimate reasons (such as if you got a job where you got your own health insurance at less cost), so there isn't anything in place to stop this, ordinarily.

Once a divorce case is pending, though, it's usually not considered an appropriate tactic;  some courts automatically rule this sort of thing out.  If your court doesn't work that way, and if you're at all worried about something like this, you can usually get a court to enter a temporary order that prevents any insurance changes or asset transfers without the court's approval during the case.