dear sketchy dudes,
if you are dating someone (especially if you actually really like her), its common decency to at least let the person you are about to hook up with know that that's the case. if you proceed with the hook upage and the girl later finds out that you have a girlfriend, then not only are you an asshole for not telling her but she is also an asshole for hooking up with a dude who has a girlfriend who she wouldn't have hooked up with had she known his current dating situation.