I am going to qualify what I wrote above by giving you my own examples:
I have reconnected with a number of girls that I once had relationships with. Some re-connections have gone "better" than others. In many cases, those re-connections have come from them to me. I was open to it and it turned out good. In the one case where I attempted to reconnect with another, the attempts (two) have been "black holed". ie. No response. Not much I can do about it, but I made the attempts.
Of the re-connects that have occurred in my life, I was 100% respectful of any R they may have been in. In at least two cases, re-connects occurred while they were with someone (I was single) and it was obvious it was an attempt to emotionally (or physically) engage me, which I respectfully refuted.
It can be done successfully and respectfully. Just be clear in yourself what your intentions are.
