Bro. Scott,
I've considered this passage over the years and agree that it is a strong case for marriage being more than a sexual act, the one thing that stands out ot me in this passage is that it isn't clear that the man and women are having a sexual relationship. It seems possible in this case that they may very will be divorced and dwelling together or never married at all and not having sexual relations.
I've seen cases where a husband and wife are divorced but still live together sleep in separate bedrooms. In the case of the woman in John 4, the laws of interpretation compel me to believe that they are not sexually active as it does not explicitly say that they are. And considering the other clear definitions of what constitutes the "members" of a man and woman becoming "one flesh" is would be presumptuous to believe that they are. In short the man referred to would most likely be viewed by the Lord as her husband if they had sexual relations.