When I saw this post on Tumblr, it was the straw that broke the camel's back.
(For future reference, I will be equating "all lives matter" with "people are great" in the Tumblr post.) I know this is not a new idea, but here's my addition to it: we all thought this whole "all lives matter" thing blew over by now, for the most part. But this same concept prevails, particularly on Tumblr. For instance, with "Bi girls are great" "People are great." It's a battle of the PCs. Who can be more PC? Does it matter? We're missing the point here. We're trying to help people who need help by being PC.
With "all lives matter," I see white people trying to metaphorically wipe some of the blood off their hands. They want to victimize themselves, so they don't look so much like the perpetrators. They don't want to accept their white privilege. White people are not the victim here. Hetero and homosexual people are not the victim here. The victim is the minority. By saying "all lives matter" and "people are great," you are belittling their suffering by comparing it to yours.
The purpose of the original poster was to raise awareness for bi girls. Bi girls suffer in a unique way, because many don't even believe one can be attracted to two genders (read: bi hate), and OP is trying to call attention to that and remind bi girls that they are awesome and their suffering hasn't gone unnoticed. By saying "people are great," you are erasing "bi" from the entire line, therefore erasing the purpose of the post. This no longer calls attention to bi girls at all, rather it's more of a casual "you're worth it" reminder, which is great too, but not the purpose of this post.
It's a minority for a reason. Don't try to include yourself in it. The majority is not the victim.