A/N: Some of these will have pictures and some won't.
...

Clint Barton/ Hawkeye

Him:

This, plus a t-shirt

You:

A pair of panties and one of his plaid shirts.
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Tony Stark/ Iron Man

Him:

He loves the way the fabric feels against his skin

You:

Tony loved it whenever you wore this to bed. He could barely keep his hands off you.
..

Pietro Maximoff/ Quick Silver

You and Pietro both sleep naked.
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Thor Odinson

Him:

It would depend on whether he was in Asgard or not. If in Midgard, he'd wear Midgardian pyjamas. If was in Asgard, he'd wear Asgardian ones.

You:

You liked to be free when you were sleeping so you wore night gowns. When Thor brought you home a few from Asgard, you were thrilled.
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Sam Wilson/ Falcon

Him:

A tank-top and a pair of boxers. Pain and simple.

You:

This. If Sam was away, you'd wear one of his t-shirts because they still smelled like him.
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Natasha Romanoff/ Black Widow

Her:

You just loved it when she wore this.

You:

It was like hers but in white.
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Bucky Barnes/ The Winter Soldier

Him:

A long-sleeved top to hide his metal arm and a pair of sweatpants. He'd even wear it in the summer time.

You:

They were really comfy to wear and they reminded Bucky of the 40's.
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Scott Lang/ Ant-Man

Him:

Cassie had bought them for him on Father's Day and he just loved them.

You:

Scott had bought these for you on your birthday and because you loved Pokemon. You wore them often.
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T'Challa/ Black Panther

Him:

He looked good in them and he loved the way the fabric felt.

You:

They were comfortable and they felt nice against your skin.
...

Steve Rogers/ Captain America

Him:

They were comfy and they reminded him of what he used to sleep in back in the 40's.

You:

Well, you were married to Captain America after all...
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Loki Laufeyson

Him:

Much like his brother, it would depend on whether his was in Asgard or Midgard.

You:

You liked to wear those fancy Asgardian night gowns he brought you.
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Wanda Maximoff/ Scarlet Witch

Her:

You thought she looked just adorable in it.

You:

Wanda got all flustered whenever you wore this to bed.
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Bruce Banner/ Hulk

Him:

Bruce had purple button-up pyjamas with green trimming.

You:

You had a green top and a pair purple shorts.
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Peter Parker/ Spider-Man

Him:

Peter had a pair of Star Wars themed pyjamas.

You:

They were really quite comfy and they weren't too revealing.