I've forgotten, I thought there was mention of an affair in season one. Or was it just implied? I do admit that later seasons played it down to simply being attraction.
In "Conspiracy" we learned that Picard met Jack and Beverly when they were already married. Later in "Attached" we learned that Picard always had feelings for Beverly, but never revealed them to her because he didn't want to betray Jack. The truth only came out because their minds were joined because of alien tech.