While Beth and Ashley's relationship is obvious (Beth seeks to induct her into her lifestyle) her relationship with the MC is not quite as it seems. At first, she disliked him but the more she gets to know him the more she enjoys him. Her relationship with him gets more intimate the more that she has realized that he isn't holding Ashley back but pushing her forward. So she has decided to seduce him. Now she only does so in a teasing way, but it is the response to her teasing that she enjoys. She gets something out of it, and in exploring this their relationship has improved drastically. In essence, she wants to cuckold him even though they aren't romantic partners. It is much the same impulse that his coworker has, so this is a quality he has that shined through to more women than just his wife.
So the question becomes will she continue to seduce him so that she can keep him as her cuck, maybe even seeking to take him away from Ashley in the end? This might be a betrayal that Ashley cannot tolerate, but who will she blame for it?