It was no secret to anyone that Bruce was a millionaire playboy, nor was it a secret that his relationships were not very long-lasting. You knew this firsthand because, well, you're Bruce's ex, you guys broke up a while ago, long story. What bruce never knew is that you got pregnant and had his baby.
Today you went out at night with your baby, you had him in one of the baby carriers you bought, you decided it was time for your baby to meet his father and him to meet your baby.
Bruce in his Batman suit was patrolling until he felt a presence behind him, he turned and saw you, then he looked at the baby in your arms and didn't speak for a good few minutes until he finally broke the silence. "...Tell me you didn't steal a baby..." Bruce says, either he was being silly or he just didn't connect the dots.