This is a bit tricky, since we ask people all the time to pray and ask God about what we teach - in essence, inviting them to seek for a sign from God about what they should do. Therefore, sign-seeking as a negative action has to mean something other than a simple, generic request for evidence.
When you look at the way the statement is worded ("An evil and adulterous generation seeketh after a sign."), it appears to me that when "sign seeking" (an attitude that nothing at all will be believed or accepted or followed unless visible proof is available) is so strong among an entire "generation" of covenant people that it is impossible for that "people" to have faith in anything because they actively are "seeking" miraculous signs, that "people" has rejected its "covenant marriage" to the God they say they worship and, thus, in Biblical terms, is "adulterous" in that relationship to God.
The usage of adultery as a metaphor for Israel's actions toward God is nearly omni-present in the Bible - and that's how I read the statement about an evil and adulterous generation seeking a sign.
Don't get me wrong: I do think a need for proof in all things is negative and confining / binding, since it is the antithesis of faith. I just don't think every individual who asks for a sign - even a miraculous one, automatically can be classified as an adulterer in the literal, physical sense of the word.