The problem I have with that is that it doesn't really let us come up with rules that everyone can agree to abide by.
That's basically the point of having these norms of public or political behavior, right? They're like laws of war - the idea is to be able to agree on how we go about fighting our ideological battles even as we disagree about whatever it is we're fighting about. Like, we all agree that you can't go around killing voters who disagree with you so that you have a majority.
It's important that these rules be ideologically neutral because otherwise they don't really have any force. For example, we really don't like it when Republicans in the Senate filibuster every little thing. But "you shouldn't filibuster good bills" doesn't help, because if the Republicans thought the bills were good they wouldn't be filibustering them. We need something like "you only filibuster when something is really, really important", so that there's a culture of allowing some bills to go through even though you disagree with them. We had that at one point, but it broke down and now we're worse off.
Specifically, the problem with "pro-discrimination political activity doesn't need to be protected" is that we don't have agreement as to what constitutes pro-discrimination political activity. I mean, if we did, then we wouldn't need the norm in the first place because if gay marriage opponents understood what it was they were doing they wouldn't be gay marriage opponents. They understand the "gay agenda" as being in part an effort to persecute Christians or convert their children, or whatever. Other people will tell you that liberalism in general is about class warfare and punishing success. Or that anti-white racism is the biggest problem we face right now. These people will sincerely tell you that they feel like we're the pro-discrimination side here. So this norm doesn't work - it just tells everybody that it's okay to do bad things to their political opponents. I think this kind of approach ends up justifying boycotts for basically every political disagreement, from both sides, when what we want is a norm that works to clear out a big space in public life for differences of opinion, even fairly important opinions.