I feel that, as a visitor to someone's blog, we should behave as we would when visiting a friend "in real life." If our real-life hostess brought up a deeply held conviction or point of view, we would assent to the aspects of it that we could, but beyond that we would keep our thoughts to ourselves. We would not tell her that "There are other points of view than yours"; or, "I'm glad I have made other choices than you have made"; or, "I have seen people who made those choices fail" or anything along those lines. We would not attempt to turn the discussion away from her purpose and direction into a forum for broadcasting our own viewpoints and lifestyles. To do this in real life would be very rude, and breach one of the most basic rules of etiquette:"Follow the lead of the hostess". It would invite a spirit of contention and strife into the gathering; could lead to hurt feelings and strained relationships; and shows a general lack of respect for the hostess and her generosity in allowing us into her home.
The only exception I can see to this is if the hostess specifically opened up the discussion by saying, "What do you think about this subject? Let's all share our points of view!"
I see this type of thing happen so often on blogs; but I would love to see "visitors" acting in a way that is respectful of the direction set by the blog owner ("hostess"). And that would mean, if I can't write in support of a particular point of view mentioned in a particular post, that I would remain silent instead, and jump in whenever there is a post that I can comment productively on, in keeping with the tone and direction set by the blog owner. Let's help restore civility to the internet!"