not sure of the level of sentience that matters here for the plant eaters. if "happy" is the only word used to describe a plants feeling, then ok, i don't think plants can be happy or feel sad if they're eaten. but, if you go a little more into what can lend itself to having a consciousness, then let's go to defenses that some plants have developed to avoid being eaten. maybe answer to venus fly traps, roses and poison ivy. sure, they're maybe not plants in particular that vegetarians eat, but by developing defense, it's it showing that they're sentient to an extent as well?
they can land a rover on mars, yet they still have to stick a human finger up my ass to do a prostate exam?! - ricky gervais