My girlfriend called me yesterday asking me what I was doing for lunch. She said she had several hours free as her boyfriend was going to a party and wouldn't be around for awhile. For some strange reason, my instincts flared up and I immediately asked, "Whose party is he going to? Why aren't you going with him?" She said she wasn't exactly pleased about the fact that it was a party his ex was throwing and that he said he would feel "weird" if she went with him.
Well, this did not sit too well with me which I relayed to her. She said she was trying to be understanding as he was upfront and honest about everything and hopeful he would make a quick appearance and leave. I told her it raised a red flag that she wasn't invited with him, in fact, asked not to come, but that it was a first time offense and perhaps a last. I told her she had every right to voice her concerns and to let him know that if it happened again, she would not be as understanding.
Overall, I think its fine for ex-partners to remain friends so long as the relationship is completely platonic and emotionally void of lingering romantic feelings. To alleviate concerns, new S.O.'s should be introduced as soon as possible to ex partners whom remain friendly in an effort to show complete transparency. Afterall, those you have nothing to hide, hide nothing.