I don't believe this question or an answer is specific to OKCupid, but that's the particular platform I have been using in case there are any answers/advice that is specific to that site.
In OKCupid sending messages is asynchronous in the sense that you might send a message to some one and they won't be notified or see it unless they independently come across your profile and "like" you back. So, I assume it is quite common to have sent multiple messages out to many people before getting any responses. And then even once you've started talking to some one, you might carry on multiple conversation with multiple people on the site at any given time, especially since you might not know if you actually want to meet even a single time until you've had a chance to chat online a bit first.
My situation: I'm monogamous, and I've been chatting with a couple of people who are also marked as being monogamous on the site, both of whom I'm fairly interested in, enough that I'd go on at least a first date with. And while there's not yet been any talk with anyone about actually being in a relationship yet, nor have I yet gone on a date with any of them, I want to be as honest and forthright as anyone would expect me to. I don't want to hurt or betray anyone's trust and end up in a situation from some sitcom where some character is actually dating multiple people at the same time. I want to make sure I've done the right things by people when the time comes. And I don't want to just prematurely cut some one off completely especially if it becomes clear very quickly that things won't work with another person.
The question: When and how should I tell some one I've been talking to on an online dating site that I'm now seeing some one else when all parties involved consider themselves monogamous? What is common courtesy in these situations? How is it commonly handled?
As soon as I have a first date scheduled with some one, after that date happens? When a second date is scheduled? When I'm more formally in a relationship?
If at all possible I'd like to do so in a way that doesn't cut off all possibility of dating the person in the future. After all, if I date some one and realize it's not going to work after a date or two, or even further down the line, I'd then be interested in continuing to talk to and possibly date some one I was already interested in and had talked to. But of course I wouldn't want that second person, when I tell them or later if I tried to restart talking with them after having dated some one else for a bit, to feel like they were just a "backup choice". And I'd prefer not to just ghost some one I was talking to and leave them wondering why I suddenly stopped talking to them.