Umm, I would say it's not just the clothing but the 'attitude' that counts as well. It's not just about the fact that you enjoy your Masters cock. I would argue that if you're going to be a 'slut', then you've got to want to relish have his cock, to thrive on having his cock. To be bold and shameless about being fucked and being taken. That I would say is being a slut.

But as others have written, the key here is communication....why not in a 'neutral zone' ask him what he expects. Perhaps even get him to e-mail you so that if need be he can think about what he really, really wants and expects of you and can then tell you in an open way.