I don't know if it would jeopardize your future with someone. It really all depends on the situation and how each person is feeling.
I use to have sex with my ex, but got sick of it because it was at really inconvenient times, and I just didn't want to do it anymore, and he basically kept expecting me to come over whenever he pleased. And it basically ruined the slight friendship that we even had.
That's my story, but that's not always going to happen to everyone who has sex with their ex. It could have a good OR bad outcome. It all depends on the circumstances. If you sit down and talk to your ex, about what you both really want out of a relationship (being sexual, or actual being together as a couple) would help in not jeopardizing whatever you want to happen in the future.