Through the hard road of experience, I have learned the proper answer to that question is "Why do you want to know?"
If it's fear of disease, I'd hand them my "clean sheet".
Other than that, if they could give me a tactful reason for wanting to know, I'd give them an honest answer.
As for asking it myself? I don't want to know.
However, I *am* interested in what they say about their ex's. It's amazing what you can learn about someone by the way they talk about their ex's.
(Note: If someone talks bad about all their ex's, I wouldn't want to date them. Odds are, it wouldn't work out and then they'd be talking bad about *me*, even if the reason we split is that I came home to find them in bed with someone else.)
Peace,
Caliann