I seriously don’t understand why is the politicised mob attempting to police what a pastor says within his own church. And these are the same politicised mobs who would no doubt cry loudly about freedom of speech and so on… looks like they believe in freedom of speech, freedom to say what only they approve of. The point of freedom of speech is not to protect your right to have a pleasant discussion about the weather or to debate over which restaurant serves a better dinner. The only reason why we need to encode freedom of speech into civic public laws is precisely to protect the freedom to say controversial things, offensive things, things which would provoke the passions of others, to which the civic law is then necessary to curb and restrain such retribution in order to protect the freedom to make such a speech or to hold such a discussion. The state does not need to protect the freedom to speak what no one will be offended with or which would not incite passions, it would be a waste of legislation and ink to protect speech which would never need state protection. But freedom of speech exists precisely to provide speech which does provoke passions, etc.
And on another point I think it is important to distinguish between the theological reasons for believers to hold to a certain point of view upon the basis of their faith, and the civic public reasons justifying that point of view in the civic realm. Most Christians for example believe that murder and stealing are against the Ten Commandments, and Christians have every right to believe that murder and stealing are wrong on that basis. However when it comes to why and how should the civic order forbid and define what constitutes murder and stealing and its criminality, we do not appeal to the Ten Commandments as a justification for such laws but instead to the public reasons of the ordering of society.
Therefore it is quite frankly ridiculous for those outside of the Christian faith to attempt to dictate what a Church can or cannot preach from within its own church and within its own faith, as if Christians cannot believe that murder or stealing is wrong on the basis of the Ten Commandments but only on the basis of some civic reasons. This is a sermon given from within the Church, the pastor is not giving a lecture in a civic forum or a speech in parliament. It would be rather odd and unfitting for the pastor to invoke the Scriptures in a civic context, but within his own church, surely he can preach as his faith dictates.
That being said, I think the civic case for the heteronormativity of marriage upon public reasons can be better made and if I may so shamelessly advertise my own contributions to this topic:
Ye (Somewhat) Comprehensive Guide to Marriage as a Social Institution (I): The Heterosexual Essence of the Order of Marriage