To play daburas advocate here, there are hundreds of games, ranked and normals that are completely unwinnable. This is a competitive game where the goal is, you know, WINNING. Why should someone be expected to stay in a dysfunctional team that has no chance of winning? It's just going to A. waste more of their already limited time. B. extend the potential verbal abuse they're exposed to. C. keep them from getting into another game that actually has the potential of winning.
There are only 2 downsides to leaving a game, 3 if it's ranked. 1. you could get banned. 2. the community will dislike you. and 3. you lose a pitiful amount of lp.
All of those downsides are either irrelevant or can be circumvented. If you get banned, just use your smurf till you're unbanned. The community already hates most people so who really cares what they say. You were going to lose the lp anyway. Hell, your honor being hit is useless since you already have to buy most things in the game anyway and the hextech crafting rewards for being honorable are so diluted with junk now that it's hardly a reward at this point and more just an early level up.
Is leaving a game bad? Ya, it's generally bad in the eyes of others, but it's hardly seen as a bad decision in the eyes of the one leaving the game since you're basically losing nothing you wouldn't have already lost.
Though in your specific case @OP, since you claim to be leaving for irl reasons, just quickly explain why you absolutely have to leave the game, the people on your team will generally understand that emergencies happen and take precedence over a game.