Mmf with your own gf can be really hot. But you have to trust and be sure of your gf and you have to trust and be sure of the other guy (which we are not in either case). In other words, in a "controlled environnement".
Only in these conditions can "cuckold" or mmf with your own gf can be hot and enjoyable... and it does, sometimes, work out. Trust me! I know!
What terrifies every guy in such a "ménage à trois", is the fear of loosing his woman to the other guy. Just simple jealousy.
Understanding this fact and being reassured of the best case scenario I would vote for it.