PAA,
I will admit that I was learning the Gemara according to Rashi (I even think the language of Rashi in contrast to Tosafos Shantz is mashma like my pshat), so I took it as a given that the Gemara’s question was on Reish Lakish as well. Hence I was bothered that according the Gemara’s teretz Reish Lakish was only talking about second marriages, this seemed dochek, and it led me to my current pshat that Zug Sheini, as used in the Gemara, doesn’t mean second marriage.
According to my understanding, Reish Lakish is saying that if a person’s wife becomes a Sotah it is siman that she became his wife not be because of predestination but because of his bad maasim. In other words she is his zug sheini (according to my reading). I don’t see why that is shver.