Thanks everyone! My gut told me the story was BS, too, but I thought I'd check.
I would argue that it wasn't the infidelity itself that did Lucy and Desi in as a couple, but the alcohol which fueled that infidelity. Lucy knew about Desi's infidelity in the mid-forties and filed for divorce, but ultimately went back to him. Desi's autobiography even alludes to a bit of infidelity on Lucy's part, and since Lucy and Desi were still close and very much in touch when the autobiography came out, I would think she would have taken major issue with him including it in the book if it wasn't true. So I think it's safe to assume there was infidelity on both sides. (Although, Desi's infidelity was to a MUCH, much greater degree.)
I believe it was a close family friend who said that, for Lucy, it was one thing for her to know about his cheating, but it was another thing to have it known. Friends said that when Desi would get drunk, he was wild and was basically oblivious to who was watching, and his cheating got much worse. When the tabloids started reporting on his behavior, it was humiliating for her. So I would argue that it was the disease of alcoholism that really ruined their marriage.
Was buying RKO the beginning of the end for their marriage? Perhaps. But friends say it was around 1954 when things started to get bad with Desi's drinking, and I believe Lucy has been quoted as saying that by 1955 or '56 they didn't even have a marriage. Maybe if they didn't desire to own the "I Love Lucy" film and decided to just play Lucy and Ricky instead of forming Desilu, their marriage may have made it. But if that had happened, the show may have not benefitted from Desi's innovation and talent as a producer, and it wouldn't have ended up the ledgendary show it became. So who really knows.