I have a group of friends in a play who have a partial training in massage therapy. One night after a rehearsal, one of the men gave me a wonderful back massage standing up (with clothes on, of course). The next night I was sore again and he repeated the massage and then asked me for one, which I gave him. However, I’m wondering now if that is considered immoral by the Church. Obviously a massage isn’t worth mortal sin!
In a professional setting, in which professional protocol and boundaries are observed, it seems fine for massage therapists to offer professional therapeutic massage services to members of the opposite sex. In an informal setting among friends, even those with professional training, it can cause needless scandal and a near occasion of sin. In the scenario you recount, I can only recommend either asking only a female friend to give you a massage – and only offer reciprocal services to female friends – or find another way to relax your muscles (e.g., a hot bath or shower when you get home).