Thank you for your question. It is a difficult question to answer, as there are several routes that may be taken, some of them much more damaging than others. First of all, it is important to realize that if you were to pursue a relationship with this individual without first working through your other relationship, than it is very likely that someone will be hurt, whether it is your wife or your massage therapist, her partner, or yourself.
One of the most important questions for you to ask yourself is: what do you see at the end of this? Do you see yourself with your wife, your massage therapist, alone, or otherwise? What is it that you really want out of this potential relationship? Do you want a long-term relationship, sex, friendship, or something else? The answers to each of these questions will lead you to a different potential path.
When we find ourselves sexually attracted to another person, sometimes it can be so overwhelming that it feels like something more. It may be a good idea to sit back and really explore how you feel about this person prior to engaging in a interaction that may end up leaving you and others feeling empty and hurt.
Another aspect to consider is that sometimes when people find themselves attracted to others outside of their current relationship it is because there is something missing in the relationship. Perhaps you need to explore your relations with your wife and determine if there is something that you are not happy with. If there is, than you need to make the decision about whether it is something worth working on in order to save the relationship or if you want to end the marriage and begin to see other people. If you discover that what you are missing is primarily of a sexual nature, than perhaps you could discuss the possibility of an open relationship with your wife. If this is something she is willing to explore, it gives the two of you an opportunity to move forward as partners.
Good luck, and please write back if you have any additional questions,
Lorin, for AlterHeroes