Nobody can force you to be rude. One can choose to say something honest but tactful.
"It's not to my taste" rather than "It tastes disgusting"
"I think you look better in blue" rather than "That yellow dress makes you look awful"
"My faith does not condone that behavior" rather than "You'll burn in Hell for that"
Or even, "I'm afraid my feelings on that subject are rather negative" which does not require one to go on and detail those feelings.
One can also decide to remain silent or to change the subject. Honesty does not require us to spill our guts. Nor is honesty the only positive value. Kindness, compassion, and empathy are signs of strength, not of weakness. So if you're "just" being honest, then maybe you're not doing enough.