A couple of questions:
1) I would say that sometimes discrimination is justified, e.g. not allowing a Christian to be in charge of the British Humanist Association or the Muslim Council of Britain. Would you agree?
I'd tend to see that as being not suitably qualified for the role in relation to what the role requires. So, rhetorically speaking, would I be discriminated against if I was rejected in my application to be the head honcho of the body that administers English cricket: I'd say not since I'm spectacularly unqualified for this role.
2) When does someone fall into the category "extremist"? Do you have some objective criteria that can be used?
No, since I think it is a subjective view that is relative to the issue under consideration although it may be the case that some scenarios are considered to be more 'extreme' than others to the extent of being legislated against. What we'd class as 'terrorism' is undoubtedly extreme when it comes to events such as those in France and elsewhere in recent times, although no doubt there may be some who see such acts differently.
In relation to SSM in the UK, on the basis of someone saying that they'd legislate to limit the legal definition of marriage so as to accord with what Hope referred to as 'Christian principles', by using 'scripture' as an authority to define marriage, then I would see that as an extremist position in a secular democracy: so not in my name thank you, since I see nothing in 'scripture' that is authoritative or binding on me or society at large.
Thankfully our UK legislators (exc. NI) took a similar view and over-ruled religious objections by legislating to remove discrimination that prevented homosexual people from marrying whomever they wanted to (provided the other person was legally free to marry).