On some international airlines, two first class seats can be converted into a double bed. It's nicknamed "snuggle class." However, mile high club fantasists should note that while some airlines have offered ultra-luxury suites with closing doors, most of these double bed offerings provide no more privacy than regular first class. From CNN:
"Covid was an epic reset button," explains Daniel Baron, a cabin designer and the managing director of airline design house LIFT Aero Design in Tokyo. "It shifted the priorities for many travelers. More people now long for things that might have seemed less important before the world turned upside down[…]
In practice, the flight attendants retract the wall to below the bed level, add a mattress pad and linens, and you have a first class double bed.
And now, the double bed is coming to business, although the design and logistics get more complicated because passengers have less room than in first — although, of course, still a ton more than in economy.
On the plus side, though, very few airlines charge any extra for couples to select the double bed over regular business class seats, so keep an eye on those seat maps! As a rough guide, business class these days is some four to six times the price of economy.