You are probably performing the rest stroke, otherwise known as apoyando, where you pluck a string and the finger or thumb continue in the same direction until it comes to rest on the adjacent string. It's a very common way to pluck , certainly on classical guitar, and is not a problem at all.
Being able to mute strings, with either hand/thumb/fingers is not a bad thing to be able to do, not so much on classical guitars, but on overdriven electrics especially, where unwanted open string vibration can become a nightmare. And that includes any of the strings - not just adjacent ones.
EDIT: with extra information...If your left hand fingers are touching other strings, it's no problem either, unless they're inadvertently touching in a way that makes them vibrate. merely touching isn't going to be a problem, in fact, with other playing, apart from scales, it's sometimes needed to press two (or more) strings with one finger. As long as the note you intend to play comes out clear, carry on.
Keep doing it, and it won't give any grief, unless while doing it, you run out of fingers for the next note!