So anyway, I got a flat in big city and the GF live 100km from here. We usually see each other on the weekend. Our relationship is post-passion, which I like, more serene. Yet, I think I'm bored and need "discovery". Don't misunderstand me, she's fine, really. She'll do anything. I'm not complaining, I just need to expand the playing field, you know? I'm bored!
Do consider your relationship in the equation. Lying and cheating: Confirmed not good, people hate being lied to and cheated on. If you don't care about her that much and figure it's fine to screw around behind her back, you will have a ****ty life after you marry her and realize you do NOT want to spend your life with her constantly up your ass. Just giving fair warning.
That said, open or semi-open relationships can be healthy. I've known people who do full-open; but more commonly, some couples actually let each other have an
occasional fling, or arrange a three-way.
That last part may be relevant to your interests. Think about this: Some guys are into double-penetration, or are okay with it, and will bring their girlfriend/wife a friend once in a while if she decides she wants to do a 3-way. Similarly, and probably interesting to you, some girls will bring a girl friend along for their man to play with (most often: girl is bi and wants to play around, and is also excited about her man joining in and taking her
and her friend).
Have you considered discussing this with your girlfriend? Seriously. It's doable. A word of caution though, if you do go that route you have to be ready to understand and accept the meaning of the word "no." It's one of those things that if she isn't kind of on the fence, thinking about it, or kinda-sorta interested, she is going to look dead at you and tell you
it ain't happening, and attempting to negotiate past that will quite literally damage the relationship. On the other hand, it may turn out she's genuinely fine with it as long as she's involved.
Oh, and don't forget that **** flows both ways. If you get to nail her and a cute friend, it doesn't make any sense if she can't get some extra action too.
Non-closed relationship dynamics-- open, swinger, polygamy, polyamory (MULTIPLE. GIRLFRIENDS. THAT YOU SHARE REAL FEELINGS FOR.), etc-- are extremely complex and can only be stable when both parties are genuinely comfortable with it. This means you need
strong communication in your relationship; be sensitive to her feelings, and sit down with her if you think something's wrong. It could work for a while and then just not be her thing anymore, and then you need to recognize that something's bothering her and realize it's time to stop and move to something she's more comfortable with.
You are opening yourself to a world of trouble. It's, however, a very rewarding experience for two people to have a relationship that suits them-- whether it's closed monogamy or something a little more open. If it works it works well, and if it fails it fails catastrophically. Good luck.