when kansa was told that devaki's 8th son would kill him, why did he imprison both devaki and vasudev in the SAME cell? the birth of krishna would've been avoided rather easily if kansa had used some common sense.

also, devaki and vasudev used to cry a lot over the murder of their children. so why didn't they just STOP having sex? rather than see the death of your child, why not just give up sex?

I think Kansa's intention was to eventually release Devaki and Vasudev after their eighth son was killed. If he didn't let them make love in prison, then how will he kill off the first eight sons?

I think Devaki and Vasudev kept on having children, knowing that their kids would be killed, just so that they can be freed quicker and so they can get to their ninth child (which wouldn't be killed) more quickly.

Krishna's story in the Mahabharata was probably the oldest example of a self-fulfilling prophecy, where Kansa's very attempts to prevent the prophecy (by trying to kill his nephew Krishna) was responsible for making the prophecy coming true.

You cant say that was his intention because Kansa immediately after the "akashvani" had occured, he was ready to kill both devaki and vasudev.. so you cant say his intention was to release both of them after killing thier 8th son. all he had to do was either put them both in separate cell\

don't try to be smarter than the prophecy dude...
that's part of the lila which has no sene of human logic...
if things had happened like you said then a lof of people wouldn't have found refuge in the Bhagavad Gita...

at that time i think also that people weren't that much focused on sex like we are now...
especially devaki and vasudev...