I’m reversing the idea for this thread.
Off the bat, I’ll say homo/bisexuality. It may take a 100 years, but eventually, it’ll be perfectly acceptable.
It’s getting there now, but both men and women showing their underwear will no longer be shocking. Women won’t sit spread-eagle in a mini-skirt, but they won’t take huge pains to cover up the white cotton, either.
Where do you live that, to you, homosexuality is currently socially unacceptable?
Earth. Gay marriage still isn’t legal in many parts.
While most of the world won’t lock you up for being gay, some places still will. Plenty of places, actually, Even several US states have constitutional amendments banning gay marriage…while the average gay person in one of those small, rural towns might not fear for his life, I bet he still feels persecuted and discriminated against.
Having children and/or living together outside of marriage.
In many areas, HOA or zoning laws forbid people from growing vegetables in their front yard. In other areas, it’s not forbidden but will at least get you a :dubious: from passersby. Let’s hope that will be innocuous and accepted in the near future. Lawns - who needs 'em? (I’d rather eat fresh produce than look at a boring, homogenous, artificially created lawn.)
Perhaps: wearing a shirt more than once? Most people I know would be horrified at the idea of doing so, but with the energy and water use associated with laundering …
i agree that lawns will be nixed soon. being fat is being more and more accepted. being spoiled also.
Texting and playing with whatever the next Smartphone Of The Future™ is in a social setting. When I’m visiting friends, there’s some that it’s fine to check your phone while they’re around, and some that it seems rude. Invariably my older friends don’t seem to like it as much as the younger ones.
Naked ladies on the non-cable TV. (for Americans of course)
Cannibal restaurants. You have to fight the other patrons, winners eat the losers.
Working part-time.
That’s socially unacceptable now? 
Cursing. Eventually people will accept that most (all) people feckin swear.
Or not working at all, hopefully. We’re basically at the stage now where automation has made employment largely obsolete.
The 1930s called; they want their utopian fantasy back.
Gay sex is still illegal here …
As in Fellatio