Need to have defined boundaries in a relationship. X and Y are ok, Z is not. Once the boundaries are set, and agreed upon by both, then it is easy to quell a situation before one person gets hurt/ends up looking like a controlling ass.
Tell her how it makes you feel, discuss some guidelines, and work out a protocol for future instances. Try to get her to see it from your perspective. Calmly say something along the lines of "Imagine if (insert name of ex she doesn't like here) were doing this with me. How would you feel about it?" And such.
And to answer the question, I am somewhat on the fence here. I feel like it is the person in the relationship's job to know the boundaries of what is acceptable to their partner and to make sure, especially when their partner isn't around, to abide by those boundaries.
I think of it in the way that, men especially, are going to try to do anything they can to get any girl they find attractive or whatnot, into bed, or at least into their "orbit." Can't fault a guy for being attracted to your woman and making advances on her, but you can certainly fault her for not immediately recognizing those advances and either gracefully sidestepping them, or flat out telling the guy to fuck off. Although, beating up someone who is pushing up on your old lady a little too hard/often can be pretty fun, too.