Personally, I also don't think that Jack married Lureen for her money. I don't think that he was that kind of guy. I think that he did it because he probably felt something for her when they first met ("I just like the direction you're goin' in"). Perhaps he loved the attention he got from her at the start~~she kind of "went after him" aggressively when they first met: ("What are you waitin' for? A mating call?") And maybe, also a little bit because it was what people expected a young man to do in the '60's...(eventually settle down and get married...) Perhaps, it was a half-assed attempt of trying to "fit-into" the heterosexual environment he found himself in.
I agree...I just don't think Jack is the type to marry flat-out for money and no other noteworthy motivation.
However, and I know this has probably been discussed elsewhere, I just can't help but wonder if these was a shotgun wedding. The details in the movie have Bobbie being born only about 4 or 5 months after Jack and Lureen meet, but I suspect that was simply an oversight in timing:
--August 1966 Childress Rodeo (It's August, and it's 1966 unless they are showing us scenes out of order because this is after Ennis blows up and the 1966 4th of July fireworks)
--Late September 1967 Jack describes Bobby as being 8 months old. That would have him born in January, about 5 months after the two meet.
Unless Jack mistakenly said "eight months' when in fact Bobby was only 4 or 5 months old, the timing doesn't work out right. (I **don't** think that this is not Jack's son. That's just too important a detail to present in a cryptic way and then go nowhere with it.)
However I do think that the shotgun wedding could be implied even without trying to do the math. The movie shows Lureen going for it with Jack in the back seat, and the very next scene between them shows Lureen with little Bobby. Dates aside, going from a sex scene to a baby scene is often, often a clear way to imply that the child was conceived in that coupling.
Jack could have easily married Lureen because of an accidental pregnancy.
Yikes...I'm getting way off topic here. Sorry!