My personal suspicion is that it's not the truth-telling that gets the LBS in trouble (long term, anyhow) so much as it is any whiff of self-righteousness or injured innocent martyrdom accompanying it. Nobody in an adult LTR is an innocent; we've all done terrible hurtful things to each other, knowingly or not. Ya gotta check the smug mode at the door and not approach your spouse like a patient, benevolent patriarch stooping to conquer. And that, too, is Christian ...
"Two men went up to the temple to pray, one a Pharisee and the other a tax collector. The Pharisee stood up and prayed about himself: 'God, I thank you that I am not like other men-robbers, evildoers, adulterers-or even like this tax collector. I fast twice a week and give a tenth of all I get.' "But the tax collector stood at a distance. He would not even look up to heaven, but beat his breast and said, 'God, have mercy on me, a sinner.' I tell you that this man, rather than the other, went home justified before God. For everyone who exalts himself will be humbled, and he who humbles himself will be exalted." (Luke 18:10-14)
"Show me a completely smooth operation and I'll show you someone who's covering mistakes. Real boats rock." -- Frank Herbert