You should have just said, "No thanks, I have a gf".
I respect her and her courage to approach me with such sincerity infront of my co-workers. Whether she had a hidden agenda or not, I'd still give her the benefit of a doubt. I'm not a passive person by any means but I'd show her my utmost respect for mustering the courage to approach with such a sincere smile

. What's wrong for not shooting her down right there by saying "No thanks, I have a gf" with a smiley but sympathetic face

?