Hmm, a girlfriend who is into some rather interesting play and a kouhai who loves you so much she wouldn't mind being second place as long as she can be with him too...What a lucky guy.
I guess I can kind of respect him wanting to just stay in a monogamous relationship with his girlfriend by him originally not wanting to do it and at the end consider trying to cut it (the 3P relationship) off...But I mean, your girlfriend seems to really like having the kouhai girl around, and you get to make them both happy, too! So, why not just go along with it, yeah? And he did, in the end, because he can't seem to say no to his girlfriend.
Like the chorus of the song says: "I'll Be Wrapped Around Your Finger." ... seems to be an appropriate (even if unofficial) theme song for Akira-kun. Well, some might say you've got the best of both worlds - girlfriend likes to *koff-koff* "experiment", even share, and a kouhai who has feelings for you that she might not mind being the Number 2 girlfriend. Do Your Best ... you Lucky Bastard!