Who says these particular people have an extreme fear of homosexuals? You're using an extremist buzzword for political reasons.
Some people do talk about intentionally targeting establishments that are doing something they disagree with, either to generate publicity for campaigning, to force the establishment of a legal precedent or to assert dominance, or some combination of those things.
B and B's and hotels for homosexual people only, gyms for women only, men can't be elected to parliament in some constituencies? Although those aren't "next" - they already happen.
There's plenty of discrimination which is routine and which passes without comment because it's normal and a fair bit more which is praised because it's seen as a good thing. Meaningless discrimination is always allowed. What changes is which groups it's allowed against and sometimes the extent to which it is allowed. Discrimination about discrimination is also normal and unending - the idea that discrimination against one group is wrong and discrimination (even identical discrimination) against another group isn't wrong or is wrong to a very different extent.
So, for example, you liken considering allowing a few odd people to allow only married heterosexual couples in their home/B&B to stopping all women voting in any elections (a slippery slope argument that the Daily Mail would be proud of), but you don't react in the same way to B&Bs or even hotels that exist specifically for homosexual customers only, or the various other forms of discrimination which are socially acceptable.
I'm in two minds about whether or not this particular form of irrational discrimination should be illegal, but I'm quite sure that allowing irrational discrimination against some groups while forbidding the same irrational discrimination against other groups should be illegal.