You are either a Troll ( search google if you do not understand the term ) or have some mental issues that you need to see a therapist about.
Being massaged by a woman is no different to being served a cup of coffee by a woman.
Seriously, I think you have deep trust issues of men if this is how you feel. If he was massaged by a man would you think he was gay ? You logic is absurd to say the least, that is why I think you may be in need of a therapist.
Have you thought that your husband may feel extremely uncomfortable being massaged by a man ?
His choice of which sex he feels comfortable with whilst being massaged,is his choice,not yours.
There are many reasons he may choose a female massage therapist.
1. He may have been abused by a male when he was younger and not told you.
2. He may think that being touched by a man whilst he is semi naked is being " gay "
3. He might just think, now hang on a second, this may be a fucking radical concept for you to take in, but may be he just thinks she is the best massage therapist he has ever had ?
For gods sake woman, if you care for your husband and if his massage makes him less stressed and more calm,loving and understanding then back off. Your possessiveness will end your marriage one day.
One last thing, if you have no evidence to suggest your husband has been unfaithful, then back off.
Please also see a therapist as I genuinely believe you have issues.