I think the rodeo where Lureen and Jack meet is in September if I recall correctly, based on the sign on the announcer's booth we see very briefly.
Actually, the sign says,
Childress County Fair & Rodeo
4th Annual Rodeo
Aug 7 Rodeo Aug 14.
I have always seen that sign and when I was trying to calculate these dates I dismissed it for the following reason: Just down the road about a mile or three, there is a rodeo rink. I go twice a year to look at the "cowboys" to get new dream or fantasy material. Outside the rink near the road there is a big sign that announces the rodeo and dates so that passers by can see it, know there is a rodeo today and come in if they want to. Inside, however, the signs advertise the next event at that rink. Not much sense telling people who are watching the rodeo there is a rodeo today. But it is a good idea to tell them when the next rodeo is. Is that what they did here? Who knows. But it would make sense. The rodeo today advertising the next rodeo in August, just three months away. Oddly, the rodeos here are three months apart also.
Maybe Bobbie was 8 weeks old?
Now THAT would make a lot more sense. Would Jack have waited 7 or 8 months to write to Ennis after the Grandparents/baby scene? Or, would he be more likely to write just after he was not included in the family baby gawking.
As for motivation, my opinion is that Lureen wasn't trying to get pregnant in the back seat but it happened. She was only too happy to do something her daddy wouldn't approve of, and be with a man her daddy wouldn't approve of. But I don't know that she was hoping the one-night-stand would rope her a husband.
Well now, was Lureen the town slut? Did she drag guys into the back seat of her daddy's car all the time? Or was there something special about Jack that turned her on all of a sudden? Personally I don't think either is true. I think she saw a guy that was broke, uneducated and a definite daddy buster and she went after him. She knew her dad would never approve of her marrying Jack so she had to get pregnant. Otherwise maybe she would have had a condom to use with Jack if it was just a one night thing?
Now I don't know if Jack told Ennis the truth when he said his marriage was normal and all. But it sure does bother me that in almost 20 years they never had any more kids and we never see them intimate with each other like we see with Ennis and Alma. If Ang didn't want to make the movie longer by adding intimate scenes between Jack and Lureen he could simply have shown more kids as with Jenny and Junior. Was the back seat the one and only time? Is getting pregnant all Lureen wanted so she would have to marry Jack and get back at her father? There is one thing I am very sure of. If she had gone to her father and told him she wanted to marry Jack here father would have said no and that would be that. Hells bells, Lureen couldn't even win an argument with her father over a television being on or off and she is going to marry Jack against her father's wishes? No, I don't think so.
And then there is the fact that the only time we see intimacy between Jack and Laureen is in the back seat. ALL the other times they act like roommates. Even when Bobby and his welfare was involved she couldn't care less, she would call the teacher some other time when she got around to it. There wasn't any love, caring or concern for her own child. And it was obvious that Jack had to do everything where Bobby was concerned, the teacher didn't like him because he complained too much. Nah, Lureen was cold as ice except that once when she wanted Jack to get her pregnant.