They can't work up the courage to voice an opinion that goes against the majority?  Should I respect that?

Yes, I agree that we should be welcoming of differing perspectives.  But your comment about secret Trump supporters not feeling welcome does bring up some thoughts about why this is a much neater place than people give it credit for:

1.  I could be wrong, but most of the people who post here have at varying times stated opinions that were contrary to the clear majority.  And I don't mean troll posting, but the presentation of an honestly held view even though it will be "unpopular".  Heck, if people can't do that, then this would simply be an echo chamber.  

2.  Forums that encourage the presentation of differing views rather than just reinforcing views are not for the weak.  As much as we may try to be respectful (and we should), facing opposition stings.  Most people can't handle that.  For those who can, putting your views forth in an admittedly messy marketplace of ideas has a lot of value.  For those who can't, hiding in safer venues is probably a better choice.

3.  I agree with the maxim that we should respect the person, but does't mean we must respect his or her beliefs.  I hold lots of beliefs in contempt, but not that many people.  People generally post that way to.  If say talking about Trump voters in general (or more accurately, their reasons for doing so), the tone can be very disrespectful.  But if debating a particular person, few are so jerkish as to display that disrespect.  So, odd as it may seem, by not standing up for what they believe, these secret Trumpettes are actually ncouraging that disrepect that discourages them.

that Trump supporters approach things like I do with my wife and teenage daughter.  I can know full and well that I'm right and they are wrong, they are FOS, or just being irrational.

I do a quick calculation in my head. Worth it? Nah. Then I just remain silent. So if they know saying they are voting for Trump will cause them a headache, they might just be trying to avoid the asspain. That isn't cowardly, it's avoiding annoying people that feel obligated to ridicule your political beliefs. 

Sometimes people want to avoid debates where there is zero chance the other side will even humor their point of view.  Sometimes people don't want to have to explain themselves to someone who will dismiss any explanation they give. Sometimes people don't like to talk about politics at all.