The Gospel today is short and to the point. The Pharisees are hounding Jesus for a "sign" and Jesus simply shrugs them off with a refusal that amounts to a "What good would it do?"
It's not that there's something inherently wrong in desiring a sign from God. In his approach to spiritual discernment, St. Ignatius presumes we will ask (and receive) a sign from God as a confirmation of a decision taken after prayerfully seeking God's will.
Could it be that the Pharisees were not actually "seeking a sign" but, rather, "setting a condition" for Jesus?