I consider it completely legitimate.

It is legitimate. But not ethical unless of course both partners agree on having a certain "freedom".

Having said that, I think loyalty is the most attractive quality in a partner and an unspoken promise in a romantic relationship. But yeah, if for some reason you are not satisfied or get interested in someone else (it's natural), be open about it, instead of going behing their back or what we say these days? "coldplay-ing" them. And expect the same from your partner. You then have no right to even get offended let alone murdering them.