I didn't catch your age group but gather you have been together for 4 years with your girl friend and potential wife. I have struggled with a libidoless wife for 10 years, but that was after 15+ years of a reasonable sex life. Based on my experience, I don't believe there is any obvious ways of turning around someone's sex drive - the professionals we have encountered only seem to offer minimal help to us. There is nothing much to lose, however, in seeking professional advice, just don't raise expectations too high. It would appear that a relatively frequent "mutual" sex life is important to you and that you would not be satisfied with the status quo of duty sex every now and again. I doubt that it is very likely that your girl friend will develop a sex drive on her own and I wonder how many years she would even want to engage in duty sex, as unsatisfying as that is to you? I would advise you to explore with your girl friend whether she is willing to obtain professional advice in this area in order that the two of you could be more sexually compatible - on the other hand she may well be of the point of view that you are the one with the "problem" and that she doesn't need to change, rather you should lower your expectations in this area.
Anyway, based on my sexless marriage for the past ten years (except the odd duty sex encounter) I would not wish a sexless marraige on any spouse that has a reasonable sex drive - it will drive you into misery and eventually end up in a failed relationship. Bottom line - I would not get married until there was a reasonable level of sexual compatability.
