Don't think of it as "controlling". All you are doing is drawing a very reasonable line. If you are getting married the vows contain the words "foresaking all others" i.e., you are number one. Not her friends. Not her ex. You. All you are doing is saying that if those vows are to mean anything she should not be hanging out with an ex.
With my ex I think the issue was an obsessive desire for attention/validation from others. One man (i.e. me) could not provide the level of round the clock attention, so she got her exes to help. Frankly that was unacceptable to me so I eventually split up. Some women have this desire for attention and unfortunately it never seems to be something women like that can let go of.
For other women, having exes around is because they still find them attractive and/or want to keep them around so they can go back to them. That is more a case-by-case issue, and there are ways around that. The easiest and fairest way is to ask "is it really foresaking ALL others", or do you make exceptions for your ex?
The wedding ceremony takes place at the altar for a reason; an altar is a place where sacrifices are offered. You are making a sacrifice, she should too. Asking her to not contact an ex is not asking for one of her kidneys, it is reasonably asking her to cut an ex out.