Assumptions are the enemy ...
I just really do not see the point of that kind of stuff. If I was married or had a GF and my they wanted to do something like that I would be totally insulted.
Then it's best to have a lover that shares your same values. Be open with her about this when you get serious so you are not surprised later. And people do change, and they don't mean it as an insult at all.
How bad does my performance in bed have to suck, if she would want to do something like that?
Rarely is it about "performance." In many cases, it's about feeling wanted and desired. And in others, its about the fact that some men and women just don't want to settle down with one man or woman.
Even if it was just suggested and never actually happened, I don't know if I would be able to still be with my wife or GF after that.
To each his own. As I said, it's probably best to bring this up before you get serious with someone. I've seen too many married people break up because one lover wasn't honest before the wedding, and feel back into what they really wanted (multiple partners) later.
Again, the answer isn't always that a woman wants someone other than you or that you don't "live up to her standards." It's really about what you and she want. If you and she agree that you want a monogamous relationship, then that's probably best for your marriage.
I'm glad I've found a lover that wanted a pure, monogamous marriage. It makes me feel the most comfortable and has the least complexity. But should she ever change in the coming decades, I am ready to accept her desires.
Yes why not, then i can fuck her off and get a younger better model
Just remember that in many US states, cheating on your spouse does not forfeit many rights. E.g., a spouse who cheats on his/her spouse does not entitle her/him to any less ownership of assets, right to alimony, etc... you have to show other circumstances than "he/she cheated on me!" Most people don't realize that many states are blind when it comes to sexual misconduct on its own, unless it actually causes harm to another party (such as children), and even that may only factor into other things (e.g., custody), and not work against their rights in the divorce.
Have several friends paying alimony right now, even though their wives totally slept around while they were saints.