I dated a guy for over 2 years and kept some of his clothes in my closet, including underwear. Washed them at my place, too.
But that was because he slept over so often, taking showers there (usually together -
), so that he needed fresh clothes on hand. Either that or drive around with a suitcase in his car. He also kept complete toiletries in my bathroom. I didn't do the same thing at his own house, however, because he was still closeted to family, afraid during drop-ins they'd catch us, or see the evidence of my shacking up.
We'd also swap some clothes to wear, similar size, but never underwear. Nor did I keep any of his things as trophies when I moved away.