Well, I'm not a girl with a bi bf but let me take a shot at this...

There is an off chance that the conversation, picture and masturbation were unrelated; men are notorious for, as the data says, thinking about sex every seven seconds. It is possible that his friend called at a bad time and he chose different priorities than you would have. Seriously, if he wasn't talking about sex at the time I would seriously consider this - longshot or not.

Second, if you are like most couples, yours is a monogamous relationship. By definition that means one person without a caveat of how many sexes you're into. If he's bi, then you should probably handle it the same way as you would if he was masturbating while talking to a friend of his that happened to be a girl, whatever that would be.