Technically you would be breaking Contract of Carriage, which is not a legal thing per se, though airline can take legal action by taking pax to court, which would be a legal action.
Perhaps the most famous case was United (IIRC) v. Skiplagged.com, where UA got spanked and Skiplagged is still going strong. I also recall Lufthansa actually winning, but IIRC losing on appeal, but that too was for missing last leg, not return journey. They did win the case against the guy buying refundable Biz tickets and using the airport lounge to get free access before cancelling the tix, but that's something entirely different
Lots of discussion about that in various airline fora, mostly under "missing last leg of journey," or words to that effect. I recall much less discussion about your particular situation, but it has come up.
Things change in three months, sickness happens, family crisis happens, and especially with a one off there probably isn't much to worry about, especially if you don't use your Frequent Flyer account. Not much point in that these days as earnings from flights are minuscule, and elite benefits are pretty limited even if one has elite status.
One thing to consider is open jaw, in case you can find a different destination where you would actually want to go (and potentially get back from).
Now that I think about it, saw that at least on some routes, United (having lifetime UA status I tend to try to stick with them for the Int'l travel benefits, i.e. lounge access) had some flights to Asia for half the price of r/t
Also, Google Flights is your friend.