Imagine this scenario: You're out on a date with some new man or women and things seem to be going well, but every once in a while, they bring up their ex-boyfriend/girlfriend, or their ex-husband/wife and all they do is complain about how awful this person was. They seemingly can find no redeeming qualities in their former partner and view them as nothing more than a huge mistake.
I generally take a pragmatic approach when I hear about altercations between two people. There are always two sides to a story, and I'm sorry to say that I won't always take your side of the story as the gospel truth. I know that in your story, you're going to omit any details in your telling of past events which would make you look less than stellar. That's fine. It's normal. But, knowing that, it means I can't take everything that you say at face value. If I were to talk to your ex, they'd likely have a completely different perspective on matters.
Hearing your stories, though, I start to take into account how much this person is still a part of your life. If you are constantly harping on about him or her, I'm going to assume that they are still a big part of your life and that you may even still feel somewhat passionate about them. Whether that passion is residual love/lust, or burning hatred, I won't really know. All I know is that they are still front and foremost in your mind... while you're on a date with me.
You might think that I'm supposed to feel like I'm way better by comparison, or you might think you're sending me warning messages of what you won't put up with. I really don't know what the fuck you're thinking. I just know that you're not really thinking much about me. You're not in the moment, here, with me.
What I really start to wonder, though, is if you'd be talking about me in this same way, if we were to date and things ultimately didn't work out between us. I don't want to think about the shitty things you'd be telling other people about me while leaving out details about your role in why things didn't work out. And this is especially the case in the gay community, which is much smaller than the heterosexual community, and there's a solid chance that you might be telling my potential future dates how horrible I am. I especially don't want you sharing my personal business which I may have confided in you while we were together.
I mean, some things that you say about your ex, like he used to steal from you, or had a bad drug habit, or drank all the time and abused you, etc., well, those are generally justifiable complaints. If your complaint is that he ignored you or that he was always distant, or noncommunicative, or any number of other concerns, I would think, I wonder what reason he had for being like that. Perhaps you're just not easy to talk to. Or maybe you can't take constructive criticisms, or you overreact to things. Maybe the two of you just drifted apart and he didn't mention it. I don't fucking know. What I do know, though, is that you won't shut up about him.
So, let's say that you and I are dating for a while and you're becoming distant from me, but don't say anything. In response to that, I might not feel quite as close to you, and perhaps we start having sex less and less frequently. Then you get grumpy that we don't fuck anymore, and you add that to the reasons you're distant. You then start feeling contempt towards me, and I feel it for you. Eventually, we end up breaking up, and you just start telling people that we never had sex. The reality of the situation, though, is that we both had a problem of a lack of communication. I should have asked why you were feeling distant. You should have asked me why we weren't fucking. We simply weren't talking, but now you're laying all the blame on me for how things didn't work out.
I think of these things while you're telling me about your ex. I am thinking about what role you played in everything. If you never take any responsibility for anything, then I'm going to suspect that you have a problem being held accountable for other things in your life. That perhaps you have a victim mindset. That's a red flag to me.
If you insist on talking about your ex, I'd rather hear about how both of you played a role in the failure of the relationship. I believe that under most circumstances, both (all) parties play some sort of role. If your ex used to cheat on you, maybe he was just an unfaithful asshole, or maybe it was because you never wanted to have sex with him, were mean to him, or whatever, and he just didn't want to have sex with you. Maybe you chose to work long hours and you and he just couldn't find the time. Not that this excuses his behavior, but it does help to explain it, and if you mention these things, it tells me that you're self-aware enough to learn from that experience, hopefully.
Personally, I'd rather not hear about your exes. Not at first, anyway. Not during the first few dates. They're just not relevant when you and I are trying to get to know one another. I want to hear about you. I want to hear about your friends and family, rather than about the guy you saw just before me. Hopefully, that guy is no longer a part of your life enough that I'll have to deal with him. If he's still a big part of your life, somehow, because maybe you're friends, well, maybe then it might be appropriate to mention him. If all you're going to do is complain about him, maybe you should wait a bit. If your ex is still occupying space in your mind, that's space that I know will not soon be space for me. You need to make room for me.
Occasional bitching about your ex... Eh, that's not too big of a deal. If you just happen to mention some little gripe about him, I don't care. If all you can do is talk about him while you're with me, I'm not so sure you have room for me in your life.
You need to get over your fucking ex before you start dating again. If you want to bitch and moan about him or her, do so with your friends. Get it all out of your system and prepare yourself to move on. Don't burden your new partner with tales of the old one. It's in your best interest to give the new guy or gal in your life your undivided attention.