Actually this one acknowledged by the MC a few times in the game. Like he said he knew nothing about his gf, was he acted rashly to become her boyfriend etc. I felt like MC feels manipulated by Alexis.
For how believable she is, what I got from her path she had feelings for MC even before he left. On the plane, first she thought he was hot then after MC told his story she noticed who he is and decided to take her chances hence miles high club scene.
Then she regretted her decision and left MC, still she could not stop the emotions so she waited for him at the airport. Second date, MC showed up unannounced to her working place with Zach all charming and all so she could not stop her emotions again. She literally ditched her best friend so she can share another moment with MC again, so they went to concert together. Couldn't tell to Erynn, could not tell to MC because everything was going to shatter. All in all, what I gathered from her interactions she is madly in love with MC, probably more than Erynn and because of that she crosses every line she has. Therefore it is understandable she never gave bj, never bring any boy to her room, acting all shy, timid after they started to date etc. All these are new for her because what she feels for MC is unique too. So, I think she is one of the most believable in the game with her sister and I saw exactly same thing happened to a friend(that shit ended terribly for her though).