It almost sounds like you're not that upset about it.  Hell, if you're cool with her having a side piece and this is just a sexual thing there are many couples that have arrangements like this.  (this usually isn't a healthy way to start such an arrangement) 

Tell her since she gets to go play with others you do as well.  Say that you've set up some dates for next week and gauge her reaction.  That you are not going to stay home and be her faithful pushover while she goes does what she pleases. This may be the reality check she needs.  She needs to know that you have options too.  She may not care either way also, which is also telling.