Shmuel said you're allowed to get engaged even on Chal Hamoed, cause maybe someone else will grab your girl (Moed Katan 18b). So the Talmud asks, didn't Shmuel say that a person's Zivug is already made before he's born? (And she's not going anywhere)? So the Talmud gives an answer.
Asks Tosfos, "why don't we say that Shmuel was talking about Zivug 2, while Zivug 1 is the original Zivug, like we said in the beginning of Sota?" Says the Maharatz Chayos in the beginning of Sota, "what are you talking about?" By Sota it's only a 'drush' so you can say whatever you want, but in Moed Katan Shmuel is saying Halacha (If you can get engaged on Chal Hamoed), and if shmuel was only talking about Zivug 2 and not Zivug 1 then he should have said so, because an Amorah always has to explain himself and not talk in riddles. And if the simple meaning is "A" then you don't say "B".
The same thing with sex. When the Talmud says that it's forbidden to have sex by the light, if the Talmud really ment that it's forbidden to ever see your wife undressed by the light then it should have said so. The Talmud does not talk in riddles (By halacha) and if it says sex then it means sex. (See Feb 12, Dec 2, 2012)