Can your university's student legal department review the NDA? In at least some universities, if a student works on corporate projects and has to sign an NDA then the NDA must be reviewed by the university first. See http://www.utdallas.edu/utdesign/students/ip-and-nda/ for an example of that requirement.
As a matter of principle, if you decline to sign the NDA, what will happen? Will the inability to attend the lecture affect your grade? Were you informed about the NDA requirement before signing up for the class?
Peronally, it stinks. You paid your money, with the expectation that you would get a full education. Consider that a student might work in the same field as the guest speaker, but in a competitive organization. Signing an NDA may be virtually impossible. While you may not be in that circumstance, your teachers can't make that assumption about everyone.
A third option is to propose redlines, a process often expedited when you provide back a signed, edited (and properly initialed) copy for them to countersign. This option demonstrates a willingness to engage. Worst case, you're in the same position as declining to sign the NDA.