I don' think being at, or remaining at, a service amounts to tacit approval of the contents of the sermon.
I rather liked one of he celebrants who used to do a lot of wedding s in the village I gew up in. (My family were involved in bellringing, so we would often arrive near the end of the wedding in order to ring for the couple as they came out of church.) He had his Wedding Sermon which he used every time. It was very useful, as it meant you could tell exactly how far through he was, and of course no nasty surprises.
In relation to warning people about the type of service, I think it is polite to let people kno, in broad terms, what to expect if they are in an unfamiliar setting. One of my cousins converted to Roman Catholicism so the ceremony was Catholic one - at the beginng, when the priest welcomed everyone, he also explained that there would be a mass, that only catholics could take communion but that everyone was welcome to come up to the rail for a blessing, regardlesss of their faith.
I've also been to a wedding in a very evangelical church where the pastor mentioned at the start that thye normally hugged or kissed during the 'Peace' but that it was absolutely fine to shake hands instead if you preferred.
I think that as a general rule, the further from the mainstream the ceremony is, the more it is reasonable for the couple to ensure that people have some idea of what to expect - this could be a brief explanation by the celebrant at the start, or it could be information in with the invitation or the order of service