Free-Love Future: Averted, mostly, even though Imperial society has long since built contraception in, made STI cures relatively trivial, and the decoupling of sex and reproduction is close to complete (most people come about either through ectogenesis or digital genesis).
Nevertheless, what is still there is the mental linkage between sex and pair – well, not always pair – bonding, and other emotional aspects, which people have decided not to edit out on the grounds that, well, it would make the whole business so much less fun. Thus, while such matters aren’t so ridiculously prudishly handled as in the West today, sex is generally expected to exist within the context of some sort of significant emotional relationship (not necessarily a marriage, be it dyadic, star, line, or helix or whatever the sexes involved – they have terms and protocols for a variety of relationships all the way from the most formal marriages down to delesessqámél, loosely translated, “friends-with-benefits”), and such relationships are normally exclusive to one degree or another, and promissorily so (i.e., do not break your word on this point).
Further, while there are no taboos about premarital sex, etc., promiscuity and genuinely casual casual sex – as appropriately defined in the light of the above – is still frowned upon; for one, from the basic propertarian point of view that a product dumped on the market can’t be worth very much, and that they’ll take your self-valuation as you give it, thanks; and for another, that, well, there is the missing component implicit in the name, and if one is just looking to get one’s ashes hauled, there’s any number of Kiss Me, I’m Virtual software packages available with all the fidelity of reality and without the trouble of having to go outside and attempt to connect with someone.
(While this is a common pattern, of course, various other species may feel differently. Void where alternate instinct-sets apply.)