It's really just people's choices. Also, be aware that 50% of some men can feel more than 100% with other men, hence why I think those two girlfriends are with the person you are describing. It may not be enough for you, and that's that
Just be sure that it's not because of what society tells you is "normal", but rather your own decision about how good having the relationship makes you feel and how much love you are giving and receiving. It would be very sad to automatically judge and reject certain options just because of other people's rules about how relationships should be like, and potentially miss out on having love in your life (no matter what form or shape it takes).
It's crazy from society's point of view, but from the whole self-improvement/PUA(pickup artist) community it's fairly normal/standard. They've went out and improved themselves in every way (physically, emotionally, mentally, financially, sexually, spiritually) and thus have a higher capacity to give and receive love than most other people who would otherwise be stuck with their own issues or blocks. Contrary to what many people think, most of these so-called "players" actually do long for a monogamous, fulfilling relationship, but these self-made men often can't find a female counterpart that can give and receive love at the same level, i.e. gotten over her issues and became a fully trusting, interesting, radiant woman.
Of course, there are still some childish "players" who think racking up notches on the bedpost or having trophy girlfriends means they're winning the game of life; those you just ignore and stay away from
Specifically to your situation about his feelings of "being trapped", I really have no idea... could be just an excuse for polyamory, but could also be legitimate, i.e. if you only date one person at a time it is very easy to become infatuated, attached, and clingy, and believe that s/he is "the one".
*shrug* hope this helps, from a male / PUA point of view
