Opponents of gay marriage did and do exercise freedom of speech in the UK - they've organised demonstrations, public speaking events, and spoken in the house of commons. Society just disagrees with them because their arguments weren't very good.
Any private citizen in the world who disagrees with homosexual marriage or adoption has the right to speak their mind, so long as they don't act on it - i.e. deny somebody goods and services purely because they are gay.
There are, however, thousands of advocates of the opposing view- that gay people should be tolerated - who are beaten, persecuted, imprisoned or even killed every single year.
Let's not have any delusions about which side is more embattled purely because we in the UK are lucky enough to live in one of the minority of societies that has had the courage and civility to treat gay people as human beings, and afford them the protection of the law enjoyed by every other citizen.