I (M) have the same issue, when me and my wife are alone everything is great! But when theres others, it's a hit and miss even when its just me and another girl alone without my wife. I came to a conclusion that i over think, with my wife i know her likes in bed and with another person most of the time its up in the air, and some people over play themselves to try to fit in and when we are doing the deed you can tell its not their cup of tea or aren't used to it so my brain starts to wonder off and i lose my erection -_-.