So reflexively, I went into my behavioral science mode and begin to wonder why.
As a matter of fact, this seems to be a common post-coital motif if the couple spends the night at his place.
One possibility is that the woman, after having mated, does this to mark the guy's clothes subtly with her lingering scent.
Here's another view: When a girl has finally sex with a guy, having done so generates a little conflict. So after she had engaged in sex, she unconsciously wants to tell herself that he is a good guy who can take care of her and has the potential of loving her. By putting his shirt on, she is in a way affirming to herself that she is comfortable with him. She reassures herself that he has the necessary qualities, habits and personality which will allow her to feel at ease with him and somehow justifies having sex with him.
Still a third possibility is that she thinks she will look cute and sexy; and possibly inspire an encore.
Finally, she dons her lover's dress shirt as a type of victory trophy. Have you any opinions on this?
Anyway, I have not found any research that specifically addressed this topic. I wonder how the NSF would react to a grant application?