Dear Sir, I'm 34 Years, Married And Have A 6

Dear Sir, I'm 34 years, married and have a 6 years old son. So far I had sex only with my wife and I'm successful in that. But last night I got an opportunity with someone else but I didn't get erection and intercourse didn't happen. I don't have any such issues while I'm with wife.This caused a lot of shame. What could be the reason and how to overcome. Please advise