My girlfriend and I have been dating for almost 2 years and have been together every moment since we started dating. Lately she has expressed her wanting to have a sexual relationship with a man she has a crush on. I don't want her to feel like I don't trust her. She says all she wants is sex with him and wants to spend the rest of her life with me. So I told her it was okay with me as long as at the end of the day we were still together. So she ended up making out with him then told me what happened. After she told me I felt like she didn't want me anymore, all she did was reassure me that she was in love with me and not him, but I couldn't help but think she isn't happy anymore. All she does is talk about this guy to me and she almost obsesses about him. He doesn't want her except sexually, after the day they made out he doesn't talk to her often, only sometimes over text. It really bothers me how he is being so mean yet she still would prefer to talk to me about how much she wants him. I don't know what to do

You are with a bi sexual female who gets a thrill from both male and female. But you got your wish, you are still together and she is sharing her experience with you. If you can't handle it, then tell her you don't want to hear about her exploits.That would be honest.

Now be aware that its usually the case that when in a mutually agreed open relationship, one party or the other is likely to feel bad, or get second thought, and be insecure. Either deal with your feelings, or change the agreement. Its up to you both to define the terms of good behavior between you.

I know you were motivated by keeping her happy and letting her explore, but now you know its harder than you thought. She is your partner, communicate honestly, or what's the point of being together?

If this is something that bothers you, you need to tell her. If you don't your putting yourself through more torture than necessary. (congrats on a 2 year relationship by the way :) ) but you guys should talk and see were it goes, it can go two ways, but you two still need to talk. She needs to know about your feelings and your side of the story. The reason why she is talking about him all the time is probably 'cause its her exploring and its new and exciting I guess... I don't know for sure and I can't talk for her since I don't know you guys. But just make your feelings known to her and talk about it.
Hope this helps! :)

Why is it, or do you think it is OK to have sex with others if you are guy, bi or lesbian?

Moral values don't stop or being faithful does not stop if you live a life other than straight.

Cheating is cheating, would she want or mind you having sex with another women?
She appears to want a open relationship sexually, which if that is OK with everyone, is done, but it normally ruins a relationship