If JKR had wanted us to think of him as a lady's man, she would have had him flirt back, or done something like have someone comment on a string of girlfriends or have an ex show up.

Exactly! It almost sounds like Harry's gotten more action than Sirius. Which (to twist a quote from a very bad 80s movie) means he was either a gentleman or a fag. Really, what are the odds? *g*