Yeah, can't take your side on that one. You're not loyal, you're an asshole and she's right- you're just doing what you want and being selfish with disregard to her. She literally told you she was uncomfortable with the arrangement. You went ahead trying to justify why and did it. You knew it was probably wrong, that it was wrong in the contract of your relationship and that is why you asked. It wasn't about honesty because you're already fully aware that it's wrong. Your intentions can be purely good but when you So established, "Hey, we are building trust like any normal couple, can you not do things that make me uncomfortable?" Then listen.

So being an INTJ, and valuing honesty and loyalty, I told my girlfriend that she was coming to stay, and asked how she felt.

Told your girlfriend she was coming to stay and then asked her how she felt. No, you should ask your girlfriend if it's okay if this other girl who she feels is not a good person, can stay in your personal space. Then you make your judgement. If you're going to do whatever you're going to do, then it's pointless to ask her how she feels about it because even if she dislikes it, you're not going to change your mind and your going to do what you want. It's insulting that you clearly don't give a fuck about the boundries you two are maintaining at this time in your relationship. Don't get me wrong, maybe she has some trust issues but I think it's more that you disrespected and ignore her.

Nothing has ever happened between my friend and I, we are just really close, but I have gone on a spontaneous 3am trip to the beach with her which pissed my girlfriend off at the time.

Yeah, you can have your friends and do whatever but I'd be questioning that too if I was her. You need to go and flip the roles here. How would you feel if she went with some random guy who you felt was untrustworthy 3am to a beach? Really, it's not that she will do anything but that the other person has no problem trying to break your boundries, your contract, and she has no issues with creating the scenerio to let it happen.

But I have never cheated on her and I am fiercely loyal to her. At the same time she told me I must do what I need to/think is best ... so I invited her to stay over then as per the original plan.

Is that really doing whats best for you?

Think you're probably a great guy who had good intentions and probably alone on this one, but because your So established their boundries, the relationship is fresh and you blantly ignored them doing whatever you want with this other person who your So feels does not care very much about your relationship contract annd who you had questionable relations with (3am to the beach with just you and her is more of a couple thing imo)- It makes sense why she is upset. Maybe not upset enough to say you're disloyal but upset.