tl;dr: Boy meets girl but girl wants someone who gives her the tingles. She can't have him, so settles.

Main Body: Another confirmation of TRP principles, as culled from the pages of relationshits. Here's the Original Post.

Our "hero" doesn't have an Instagram account but his girlfriend does so when his cousin asks to see pics of her, he searches her username and stumbles upon a site where she details all of the sex she has had with the guy that she actually wants. When he didn't want to commit, she drifts into a relationship with OP and when OP wants to commit, she goes back to the Alpha and gives him one last chance to date her.

AF/bb in full effect. A post CC girl settling for the back up plan because she's fast approaching the wall and is losing her appeal to the guys that she actually wants who just see her as a pump and dump. Also, the fact that she gave the Alpha the ultimatum just shows that she tried to branch swing to a guy with a higher SMV than her loving (soon to be, but not yet official) boyfriend.

and she wanted advice on how to do anal with his big dick, however when I have slightly brought it up she tells me that her butt is exit only

Fucking ouch. But yet another TRP theory confirmed - if she's into you, she'll let you do whatever you want to her. If she's not, you'll get a starfish in bed and, eventually, a dead bedroom. If she's not attracted to you, she'll go drier than the Sahara once she's secured your resources.

And here's the Update

OP goes to his cousing advice and gets more confirmation of AF/bb:

She described guys like cars, some are safe and reliable, some are fast and fun. My girlfriend found a car that she couldn't afford, (the other guy was out of her league), she got to test-drive with him but couldn't keep him. So then she got with me, a good car, but not the flashy/fast one

Just like a guy who used to drive Mustangs(fast sports car) when he was younger, isn't going to be thrilled about a minivan

And this, gentlemen, is something else we keep saying: in order to create and maintain attraction, you have to present yourself as an exciting prospect. Be an interesting person to maintain interest, remain attractive to maintain attraction. You don't necessarily have to be the "bad boy asshole" to get the girl, but if you're the boring, reliable slob waiting dutifully at home with no plans or interests than she's going to lose interest in you. Get off that couch, turn off that Netflix, put down that game controller, get out there and fucking do something to enrich your time and hers. Then give her a damn good fucking that leaves her unable to walk.

Lessons learned: Be the Mustang, not the minivan.