“Amendment I. Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances.”
That last bit there; “the right of the people peaceably to assemble, and to petition the government for a redress of grievances” along with that freedom of speech bit is the right to protest. The 1st amendment acknowledges this right.
The people protesting now are NOT “sore losers” or “children.” They’re not wrong for protesting, nor should they be belittled for it. They’re participating in the political system, just as is supposed to happen. They’re protesting for many reasons, chief among them I imagine the policies they believe Donald Trump will enact based on the campaign he ran. They have the right to do this!
I haven’t personally joined protests in the wake of the election because I personally believe it is more effective to protest policy itself rather than election results when policy has yet to manifest. But that’s me. Though I hope to be proven wrong, based on the campaign he ran I believe there will be plenty of policy worth protesting.
In the meantime though, to those who are protesting, since you clearly believe this is worth protesting now, keep doing what you’re doing, and don’t let others dissuade you just because they disagree. We all need to be more engaged in politics, and the way to do that is by peacefully expressing your beliefs through whatever avenues are open to you.