Threesome

I am sure that ANY guy would happily join into a threesome (2 girls and him). But then it occurs to me that there has to be someone who wouldnt want.

Which confuses me further. What non-gay guy would refuse a threesome??? Especially if lets say his gf was in on it.

A guy who is afraid of what his g/f would think of him.
A guy who actually loves his g/f and could never be intimate with another woman.