There are people out there that don't care. I've had 3 intimate partners since my surgery. One was my husband (we split for reasons unrelated to my ostomy) and two boyfriends. None of them have minded my bag. When it seems things are headed that way I tell them what's what and reassure them that it's not as gross as they might imagine, the bag is small and opaque, and if we are doing it right the bag is the last thing we are going to think about. Lol. Plus, once things get heated so to speak, my output stops. So as long as I make a quick trip to the bathroom my bag is empty during sex anyway.
I think the key is being open and frank about it. You need to set the tone and when they see you're not self-conscious about it (which you shouldn't be because hello, it either saved your life or gave you your quality of life back), and that you're comfortable and willing to give it a go, it relaxes them. Sometimes they are even curious. Lol. And I tell them if the idea freaks them out, we can give it a shot and if they can't get past it, fair enough. It sincerely has never been an issue for me.
And I agree with some of the sentiments here. If someone can't get past it, then they aren't looking at the whole picture. Basically, they aren't really seeing you - your sense of humor, intelligence, confidence, sexiness, the fact you're fun, etc. And if that is the case, your ostomy just saved you from making a big mistake - becoming emotionally invested in the wrong person.