What turns me on about having my wife let me cum on her clothes is that it is a very spontaneous thing. She's dressed and kneels down to suck my cock quickly and doesn't care that she's making a mess. It is best, of course, if it is a spitty, slurpy blowjob where she lets me cum all over her and the sperm ends up in her hair, on her face and on her clothes. And the cum shows up so beautifully if she's wearing something dark or something that looks different when it gets wet.