I have no idea how to start that discussion. That's not something I'd ask for, I'd imagine that to be a huge turn off for a chick. The way I approach this is girls know I'm hooking up with others, but I never throw it in their face. On top of that, girls know they're free to do what they like, but I'm intentional about making the sex really good for them. Truth is, (in general) most chicks don't actually hook up with other guys much if they're really attracted and into the sex. That said, I usually notice the window to be between 3-6 months that a chick will ask for more exclusivity or commitment. I have a speech I give then, and a lot of girls do opt out because it begins to get emotionally tolling for them to know Im fucking other people. Some stay. I think what's important is accepting that you can't control what she wants or how she'll respond, and setting clear boundaries about what you want.