"I have seen the ebb and tide of human sexual morality over the eons, Keith. I have seen sex given as freely as a smile, and I've seen it guarded as if it were more precious than peace. I've known people who have been celibate for a billion years, and I've know others have had more than a million partners. I've seen sex between members of different species from the same world, and between those who evolved on different worlds. Some people I know have removed their genitals altogether to avoid the issue of sex. Others have become true hermaphrodites, capable of procreative sex with themselves. Others have switched genders -- I have a friend who changes from male to female every thousand years, like clockwork. There have been times when humans have embraced homosexuality, and heterosexuality, and incest, and multiple concurrent spouses, and prostitution, and bestiality, and sadomasochism, and there have been times when all of these have been abjured. I have seen marraige contracts with expiration dates, and I have seen marriages that last five billion years. And you, my friend, will live long enough to see all these things too. But through all of it, there is one constant for people of conscience, for people like you and me: if you hurt someone you care about, there is guilt."
--Glass, in Starplex, by Robert J. Sawyer.
(To know who Glass is, and who Keith is, and why Glass is talking about eons, well, you'll just have to read the book.)