It gets pretty cold in space, and sometimes the Falcon's heating breaks. Just after ANH, Luke spends most of his time with Han when they're travelling to and from Rebel bases and such.

One night Luke is just SO COLD and Han puts his arm around him, and then they end up falling into bed together, not to have sex then, but just to cuddle to keep warm. It keeps happening, until one night they kiss and all the rest of it.