Find out what the hurdle is that is causing her to refrain from going further. It can be anything including her parental upbringing, religious ideas, stories she has heard from other women, shyness, concerned she wouldn't look attractive to another man, or just doesn't know how to go about it. I used to tell her stories when we were in foreplay...maybe something I had read or just my own imagination...just getting her hot thinking about someone watching us while we were having sex or a threesome with another man or woman (finding out if she is bi) etc etc...you can get all kinds of ideas just reading about people's real adventures here....so over time the idea of having sex with another man was not foreign...indeed it was exciting to her...then watching some movies or if she is a reader getting her some erotic literature that involves cuckolding,,,,just planting the seed....and then when we were out and we saw an attractive guy you ask her if he is the kind of guy she would like to have sex with...or maybe she has a crush on a co-worker, boss, neighbour, friend, acquaintance, relative etc. If she is into it you will get positive feedback....if she is not you won't. If she is not interested then it will just remain your dream....