At a house party at a friends once...
His girlfriend then kept bragging she gives the best blowjobs ever.
Later that evening he had to go grab some more beer as we ran out.
I looked at her and said, well suck my dick now and show me how good you really are.
At first she was a little nervous and stuttered. I replied to her with something along the lines of 'you're gonna suck my dick, eat my cum and then kiss your boyfriend when he comes back.'
That's how I discovered she was a submissive slut. her blowjob wasn't the greatest. But the overall evening and her listening like a little slut and submitting to me turned me on.
Since then I love fucking other men's girlfriends and wives.
Oh and she wanted it kept secret.