This seems to be an issue you find particularly concerning so i will try to breakdown my response in the hope of better answering your questions.

why do you think that im in love with only the idea of him?

Because you yourself said that you don't really know him - love is a strong emotion, one that usually requires strong reasons, do you yourself find it realistic to say that you love a man you hardly know?

what if he is really the great man that he seems to be?

But the point is you don't know that, and the likelihood is that you'll never know - he's in a relationship with someone else.

i dont think that he is faking the way that he is, and i can tell that he is really a good person

This statement is contradictory, you don't 'think' he is faking the way he is yet you 'know' he is a good person - it doesn't make sense. No one is saying he is fake although if he isn't on more personal terms with you he is likely to be courteous and polite etc.

i know that i would love him "for real" if i just had a chance with him

Again this is another contradictory statement - you cannot know what you have not experienced, you don't know enough about him, you also have limited information on his perception of you.

i know that he will have "flaws" but i am sure that i can overlook them
im not expecting him to be perfect and even now there are some things that i do not like about him but it doesnt matter, to me he is already perfect and i love him already

More contradictory statements. The last part is particularly odd - you're original question was whether you could love someone you don't really know and yet you seem to have answered your own question. Perhaps what you really want to know if what you're feeling is somehow 'wrong.'

i cant really imagine what he could do, besides something like cheating on me, that would make me not love him

How do you think his girlfriend would feel about him cheating on her with you though? I admit i find it strange that you would happily be with someone as long as they don't cheat and yet they would have to cheat in order to be with you?

None of this post was meant negatively and i hope you don't take it that way. I believe your thinking is actually quite clouded at the moment and perhaps you might benefit from getting some sort of professional help if you find this situation begins to affect how you function from day to day. People here can only help encourage you to find healthier ways of dealing with the situation should you desire this, we are not professionals, nor can we give you correct answers or tell you what is the right thing to do but we CAN be here to support you if you so wish. I hope there was something to take from this response. All the best.