I`m not male or gay so you might not want my input but here goes anyway.
I would not allow another woman, or a man that I was not having a sexual relationship with, masturbate me for two reasons. Firstly I can do the job perfectly well and probably better than anyone else, and secondly I would probably feel inhibited and therefore find difficulty in climaxing with someone I wasn`t in a relationship with.
Would I want to perform the task for someone else? Thats difficult to say really. Deffinatly not to another woman, probably not to a man I wasn`t involved with. Would I want to watch someone else masturbate? Not female, but there are some men or groups I would like to watch, I think I may also like to watch my boyfriend being masturbated by or masturbating another woman, but this is something we have never explored and are unlikely to explore.
What if I were unable to do the job myself? As I am sure there are people who need sexual release, but are unable to do this for themselves, disabled people for example. I still think I would find it difficult to allow a woman to touch me, or allow a man who I didn`t have that sort of relationship with to. Then again there are lots of things I would find difficulty in allowing others to perform for me, but sometime it might be necessary. Is masturbation that different.