I don't think this is about protecting marriage, because marriage is far more threatened by divorce and the feminist ideology that women are superior rather than equal.
This, IMO, is a slippery slope issue protecting religious freedom. Sure, what they are asking for now is moderate: give gays the same rights as straights. Most people do not care what gays do in their home. Most people don't even care if gays can adopt, as there are plenty of bad straight homes as well. The problem lies when it stops becoming the dictionary definition of tolerance (live and let live) and becomes the liberal difinition of tolerance(forced acceptance and support).
If we look at sexual choice (and it is a choice) as the same as race, we will start to view any resistance to it as racist and inexcusable. A photographer who refuses to take pictures at a gay wedding is not standing her ground, she is a bigot who must be forced to overcome her predudice. A church which refuses to perform gay weddings will be forced to do so, just as churches were at one point sued and forced to marry biracial couples.
Note that other choices society makes, such as religious denomination, are not legally held to that standard. If you are a methodist, you will never be forced to perform communion, and if you are catholic, your church will never be forced to marry a jewish couple, if you are jewish you will not be forced to observe an Islamic holiday.
We do not, however, allow discrimination based on religious denomination. We do not refuse service for non-religious things such as breakfast, or banking or education. We already do not allow that against gays as well.
If we could guarantee this would be observed for the choice of homosexuality, then I think most conservatives would fine with gay marriage. Not all, but most. We all know, however, because they have already tipped their hand, that they will not stop there. It will not be over until gay pride is celebrated throughout the country in every walk of life. It is not about personal rights, it is about forced acceptance. THAT is the main issue.