Most Helpful Guy
Yes, it's possible, but it's the exception rather than the rule. If a guy loved a girl, but maybe he had some hot girl throwing herself at him when his girl wasn't around, and it a moment of weakness, he got with that girl. In that situation, he would realize that he messed up, and would probably confess to her out of guilt. He definitely wouldn't repeat the mistake, and he'd feel like crap about it, even if she never found out. That might be the kind of thing you could forgive.
But if this is an on-going situation, then what it means is that, despite any feelings he has for you, he puts his own desires ahead of you and probably ahead of everything else; i. e., he's selfish and self-centered. That's not real (romantic) love.
Most Helpful Girl
If you bought a pet, whom you grew very attached to, and you lived daily with it. It became a part of your life. If it were a dog, you'd take it for daily walks, give him a good rub when he asks for it, and cuddle when you feel like having some warmth. In other words, you built a connection with the animal. Now let's say one day you just wake up, see another cuter breed and decide that you want that one instead. What do you do next, you may ask. You simply buy that new, cute puppy, brings it to your home and completely neglects the other one you had to begin with.
I know it's quite a far-fetched analogy, but I want to put things into perspective to you. Bringing another dog home doesn't mean you still don't care for the one one, because you do. But the priority in your life shifted from the old dog, to the new one. Your boyfriend might still care for you, but he certainly does not love you or else you would've been so important to him, he'd never consider being with someone else.