If Laureen had not been as up front and agressively cheeky with Jack, then they probably would not have even got together. Laureen obviously saw Jack look over at her, and maybe recognised that if it was going to happen it was going to be up to her to make the first move......she may have thought he was shy or even intimidated by her, and he probably was.
I think Jack was caught up in the fact that someone actually was not rejecting him, someone was showing some interest in him and he was enjoying the experience....
He probably would have preferred it, if this interest had come from Ennis, or at least one of the guys in the bar, but so far that had not happened, so maybe it was time for him to take what was offered to him and try to enjoy it. His lack of experience with women was counteracted by Laureens boldness and it was clear to Jack, that if he went with Laureen, that she was capable of leading him and showing him what to do, and how to do it.
If not for a woman like Laureen, Jack would probably have not even linked up with a female, let alone got married......Jack was probably never going to make an advance to any woman, so when Laureen came along, he jumped on the "roller coaster", she obviously offered him a somewhat easier way of life than rodeoing, and she fulfilled his emotional need to have somone who cared about him.....and he was able to lead a life that was accepted by society. (Isn't that what is happening even in these times, when gay men get married and try to live a normal heterosexual lifestyle?)
I do not think Laureen was slutty, she was a flirt, and pretty sure of her sexuality and attraction...and cheeky enough to act on it........some men may have been intimated by that....Jack, I think, was overwhelmed and humoured by it.
I do not think she was a Lesbian, there is no indication that her sexual preferences were towards women, and just because a woman is forthright and dominant does not make them a lesbian.
Whether she was pregnant or not?......I dont think she was, even though the movie timeline indicates differently....I think the writers miscalculated by a few months there, in their intention to show how quickly it all happened for Jack.....there he was a single rodeo guy, trolling the bars for a man, no real future, alone and rejected.......then within a short space of time, he was married, with a baby in a job selling farm equipment, leading a socially accepted heterosexual lifestyle....like I said earlier....he jumped on the roller coaster......