damn.... your just going have to be straightforward... after eight year i'm sure you could do that... it never good if two is in a relationship and one is not being real about how she/he feels... go for it in the smooth way, let me know what happen too plz :-) ((its good to learn))

We have a lot of good articles on this site on the subject. In the end, I would say, be open, respectful and let her know you are not dissatisfied with her, you just want to try something new. I would read through some of the articles, they give pretty clear arguments for open marriage.