You touch upon some of the appearing randomness of lots of protestant theology, at least today.
This so called randomness wasn't as big of a theological problem back when all Christians shared the basic ideas of how to live a moral christian life, but now it's different.
The problem with liberals theology is that is argue from a position of emotions and it make use of people's lives as an argument to undermine Christian ethics and moral.
By emotions I refer to this kind of argument : how can you be so coldhearted as not to accept these two mens love for one another. Why do you hate love?!
The above is an example of arguing from the point of a given persons life. Often times liberals add a tad of absurdum and a tiny bit of personal characteristics and voila you've lost out, but on the wrong premises.
To answer you're question, in Protestantism it's easier to mislead the faithful gravely than its to mislead us Catholic due to our weight on the magesterium, the catechism and counciliar theology. Even in Protestantism one should weight the new doctrines to those of earlier times.
If Luther left no room for homosexual weddings then Lutherans should feel obligated to follow his line of reasoning.