The thing is, and I do remain suspicious of this whole thread(s), that Cas' original question was loaded (or at least a confused paradox). The question asserted that she didn't feel any guilt and couldn't understand why.
Some took time and effort and tried to provide some moral issues on the basis of monogamy and respect for the male/female unit, others said she was just having fun so why not. WM's contributions ( I must admit) made me laugh. Sorry. Slap my thigh Ed.
So going back to the original question Cas - why SHOULD you feel guilty? If you an answer that, then you have your own answer. And in response to your picture post, as attractive as you think you are, you are really really really not my type.
Now I have spent far to much time and effort on this tat, So I am off for a lie down and a G&T.