My thing has for many years been to tell girls something along the lines of,"I am the one and only cock in your life, no exceptions, no excuses... but if you want a girl too, that's fine. I'm past being impressed with threesomes, so I won't touch her unless that's what you want. I only ask that I meet the other girl so I know who she is in case she turns out to be a fucking psycho."
This works for me, although granted that I am not the typical male.