Door into Summer.

"suicide is such a final experiment, even if the circumstances are mathematically intriguing"

It saddens me that I am now very close to having read everything heinlein ever published. One day there'll be no more.

I think I agree with Floyd C. Gale's analysis, "Heinlein paints a detailed picture of both civilizations, so evocative that 1970 emerges clearly in the reader's mind as the old days,
and pretty primitive at that". It's a fast, fun, tinkery story, and although the romantic aspect gave me pause for thought, it does seem silly not to acknowledge that custom more than anything else dictates sexual relationships and of course Bob's a man for pushing at any boundary when he gets his eyes on it. If you are a Heinlein fan I don't think you'll be disappointed.