I have had the same problem, whether my sims were ex's from marriage or relationships, the "going steady" would never show up. I had made this sim girl and her relationship ex was Christopher Steel. She got together with an ex maid that I had move in. Option for "going steady" never came up, for as long as they had been together. He turned elder and I wanted her to have one more baby and they were doing their romantic interactions, he was doing the asking and it came up "break up with rebecca", I went excuse me, lol.......I guess she had gotten a girlfriend somewhere, not sure where since I play this couple all of the time, anyway, I had her break up and then all of the options came up, "go steady, engaged and marriage", so I finally made an honest woman out of her. That is the first time that had came up out of all of the sims, so maybe something is fixed, going to try another couple and find out.