You have a big problem. You are in a relationship where the two of you want different rules. Her rules are fine and your rules are fine ---except they don't match.
She obviously wants a far more open relationship than you do. It also sounds as if she really isn't ready to make the same kind of committment to you as you want her to. She is not ready to say you are the one ---- she is saying ---- you are the one unless someone better comes along. She is always checking over her shoulder to what else is available. She flirts in part because my guess is that she is a flirt - and flirting can be in total innocense ----IFshe is in fact totally committed to you - which she is not.
So the two of you need to sit down and talk and establish agreed upon ground rules for your relationship.
And no - don't say anything to the guys. She is inviting their comments, responding to them - she is the one who has to stop ( if that is what the 2 of you decide if right). Then if they do not stop you can step in - but right now it is her call not yours to make.
Susan