While there's no real logical or justifiable reason to have an affair, I've been wondering why Erin would go down this path. Everyone described her as sweet and shy, JC as the perfect husband, and the two of them as very much in love for several years. So why??? It's not like JC was a prize by ANY stretch of the imagination! We've discussed some possibilities: extremely young, away from home, brains don't fully develop until 25, etc., but there had to be something that made her see CL as a better catch than JC so what was she really thinking. (Most stupid decisions at that age involve the "thrill" of it, but that doesn't sound like Erin.)
What if Erin saw CL as her "ticket" out of the Marines lifestyle? His time was almost over and if she convinced him to leave his wife for her (possibly the pregnancy wasn't real, but a ploy???), she would be leaving 29 Palms very, very soon. If this was her line of reasoning, it might have influenced her state of mind and made her take risks she wouldn't have otherwise taken. It's easy to think the ends justify the means when you feel trapped. Then anything seems "worth it" just to get out of the current situation.
Just a thought, and ultimately I'm not sure it matters, but I wanted to throw it out to see if anyone else was thinking along those lines???