Relationship with ex is overfriendly

My boyfriend remains friends with a lot of his girlfriends - this one in particular irks me though. They will call each other, not regulary but once a month or so. What annoys me is that if he e-mails or Face books her he calls her sexy and says things like "love you to" this being friendship love. I feel this is a bit of a slap in the face and totally inappropriate - should I be more understanding?