Mature is 'and then they made love.' Explicit is 'and here's how they did it exactly.'

To wit: mature.

He looked at the envelope, spread out before him.

God, he'd never been this hungry.

Could he be gentle enough? Slow enough? He didn't want to damage it, didn't want to do anything he'd regret... but no, no, it seemed the envelope wanted this as much as he did. It slipped into his hands, it folded as he asked. When it was time for more, the card was waiting, and he somehow knew exactly what to do. He moved with his correspondence in a dance as old as the mail system, and when it was over, he was smiling and the envelope was completely, thoroughly sealed.

Explicit:

The envelope waved its flap in the air slowly, gently, and he could see the faint shimmer of the adhesive traced along its fold. It was like a taunt, a dare: won't you? And he would, oh, God, he would, lifting the envelope firmly to his lips, licking slowly at first, then faster, more firmly, tasting the envelope's essence, the faint bitterness, the sweetness to follow--

Oh, he couldn't help but smile at how it felt in his hands. It was so perfectly folded. Its paper was rough against his fingers, and its crossed folds shifted slightly as it opened for his eager tongue. Yes, yes...

Now the card, and his hand trembled as he lifted it, as he held the envelope, stretching it wide. Would it fit? Oh... oh, yes, it would fit, it slid in smooth and quick and filled the envelope to bursting, oh, made for each other, and he smiled in delight at how perfect it was.

He was ready. Now, now, now: with one swift movement he folded the flap over and he pressed, yes, he pressed the flap down and it stuck, God, it stuck perfectly, and he closed his eyes in bliss.

Afterwards, he stroked the envelope, and thought about addresses.