The words "sanctity of marriage" in response to the idea of swinging imply, to me, that your wife holds a core belief, probably ingrained by religion, which will supersede any fantasy you throw out. As long as your wife believes that there is something wrong with sex outside of marriage, it probably won't happen. You cannot force a spiritual evolution. I agree with one of the previous writers that the only position you can work from is one of respecting her stance. You don't have to agree, but you must respect. If you can fully respect her and the boundaries that you began your marriage with, then she may get to the point where she feels comfortable enough to consider playing outside of the box, or she may not. If you wish to keep your marriage intact, then you must settle for that.
On a side note, if your wife has no interest in swinging, why are you browsing swinger sites? Aren't you sabotaging yourself by focusing on something you can't have?