Complex sentence: He married his first wife, Emily, in the 1930s and, while married to Emily, had an affair with Sylvie Kosloff, who, unbeknownst to him, became pregnant.
Split into simpler sentences: He married his first wife, Emily, in the 1930s. While married to Emily, Mac had an affair with Sylvie Kosloff, who, unbeknownst to him, became pregnant.