Sometimes it's really difficult to hold your nose while you vote.
Some people are pro-RKBA and are also pro-life and anti-same-sex "marriage." That's just three parameters.
Who should someone vote for who is pro-RKBA, pro-abortion (or at least not anti-abortion), and pro- or neutral on same-sex "marriage"?
How about the person who is anti-RKBA, pro-life, and anti-same-sex "marriage"?
Toss a couple of other issues into the pot and it becomes complicated. Whatever your combination of views on a mix of three or four or five key issues, it's probably impossible to find any candidate who supports all your views. So ... which one or ones are you willing to compromise in order to vote for a candidate who has a decent chance of beating Hillary or Bernie?
Or, what if you do find a candidate who is in lock step with your beliefs and positions on all the issues? And his name is Ron Paul or Rick Santorum, and he's polling in the low single digits. Dilemma: Do you vote for the guy who best matches your views but who is guaranteed to lose against ANY Democratic candidate, or do you hold your nose and vote for the Republican who at least supports some of your views (maybe) and who has a realistic chance of beating the Democratic candidate?
As I get older, I try to be more pragmatic than I have been in the past. My overwhelming desire at this point is to take the White House away from the Democrats. If they win, they'll stack the Supreme Court, and that's going to negatively affect any number of issues I care about, so that's what is going to drive my vote this election. I don't like Cruz, I don't trust Cruz, and I don't want Cruz to be President (or even dog catcher). But ... he's not a Democrat, so if he wins the Republican nomination, I'll hold my nose and vote for him.
(And this is a big step for me, because a number of years ago I vowed that I was done voting against people, and that I would henceforth vote only for the person I felt was best for the office. Which, in at least two presidential elections, saw me doing write-in votes for myself. I still don't like the fact that we are so often reduced to voting for the lesser of two evils -- which, by definition, means still voting for evil -- but pragmatism tells me this is not the election in which to stand on principle.)