Couples back then didn't have premarital sex. So, I don't think there was a couple on it that just had sex in the car. (Also, no one was permitted down there, and it WAS monitered) People didn't have sex the same way we do now.. married people didn't sleep in the same room, let alone the same bed. It was customary for the wife to go into the husband's room when she wanted sex.. that's what Cal meant when they were having breakfast and he said, " I had hoped you would come to me last night."... To which she replied, " I was tried...." So, so on.. we know the scene. (and actually, I guess they wouldn't have even done THAT, since they weren't married yet. But, the point it, that's how it was done. Not until the late 60's did married couples sleep in the same beds.... Even on the show I Love Lucy they were shown in different beds....although, within the 50 years from the sinking to that show, marriage had progressed form seperate bedrooms, to the same room, just different beds. )

It doesn't have to have a love story... at least not like the one in the film.
I think the true love story is of the actuall passengers... the Strauss's who died together because the wife wouldn't get on a boat without her husband.. naturally he wasn't let on one.
The Astor's, who were newlywed's that weren't able to even raise their unborn child together because he perished.

There wasnt a love sotry like Jack and Rose, Cameron himself said he made that all up to draw hopeless romantic's into the story line. I think he did a horrible job telling a GOOD story line.. one that showed the actual passengers and heros as opposed to the fictional ones.