If you're "maintaining inappropriate boundaries" then it's not exactly a mature approach is it?
I suspect our definition of inappropriate boundaries differ. I used to think that the hard line was physical contact. Now, I realise that talking to an ex about my emotions or my current relationship is inappropriate because I'd be talking to the wrong person, giving them personal information about my partner and connecting with them on an emotional level. Worse still, it's withholding that information from the person who most needs to hear it and be given the chance to respond honestly.
If the OP's exes are over friendly, providing a shoulder to cry on and available for hook ups then, no, it's not appropriate for them to be friends.
But that's a huge assumption to make on the part of the exes. Yes, some of them might be fine and completely over it but this certainly isn't a universal condition and shouldn't be assumed. Yes, you need to trust your partner in this too but if an ex is there, being the chilled, supportive one, while the relationship partner's struggling and in emotional need, then this can be very damaging having this easy fallback and comparison.
But I think it's a shame people think it's impossible to be friends with exes. It's pretty normal in my friendship circles. None of my exes are a "shoulder to cry on" and I wouldn't go anywhere near them sexually, that's done and dusted! There's no sense of unfinished business.
It's not impossible to be friends with exes - that wasn't what I said. But having a harem of ex-girlfriends hanging around on the hook, ready to jump in whenever it's needed - that's an intimidating situation.
I wonder, the people saying you can't be friends with exes, are the vast majority of your friends female? Do you have many male friends? Or do you feel relationships with men and woman always have a sexual element, below the surface?
I used to have lots of male friends - couple of them were exes but mostly just guys at work who I got on well with. Never needed to define the relationship because I thought it was obvious and I knew it'd never go any further physically.
Except - I made the mistake of assuming they knew this and they felt the same. I had multiple miscommunications and awkward situations where my "friendliness" was taken as "she's clearly into me". I've had friends tell me their wife didn't understand them and offered them advice on their relationship, only to be told "you understand me, I wish she could be more like you". One guy even approached my DP to tell him that I was a prick tease and liked leading guys on and I was no good for him.
After this happened a few times, I realised it wasn't happening for any other reason except my poorly-defined boundaries.
Post-realisation, any guy friend who tells me his wife doesn't understand him gets a "yeah, your problem is that she understands you too well" or "believe me, I'd give you far more crap than you're getting from her in the same situation".
Ultimately, the golden rule DP and I use for dealing with friendly exes / opposite gender friends is this: we are a unit. They're either friends with both of us or they're friends with neither of us. If I'm replying to a message that I'm uncomfortable about my DP seeing, then it's not appropriate. If his exes are messaging him directly and wouldn't want him to share their communication with me, then it's not appropriate.
@wakemewhenitsallover - if you've got all this stuff locked down and there's no issues with your DP and exes all being together, then that's awesome - but it took me a long time to get to that place. Boundaries are incredibly important but some people find them difficult and if so, the guidelines need to be firmly established early on. Too much muddying and blurring of the lines with people who may not be as emotionally stable as you, can lead to pain for everyone concerned.