That is a bit to selective on a general problem. The trashy girls are usually loyal when they find a worthy guy for them. That is usually not the issue. The problem is her having an awful morals and slowly showing him them. That allows him to relax his morals enough to over look her awful behavior or just ignore it. Usually his friends are the first to notice and say something. She gets rid of that problem by isolating him by smithing him in love. She only let's him talk to the people whom she knows won't have him question her. In the end she slowly shows herself with him duped enough to accept her for the filth that she is. Yeah I lost a good friend to a bitch recently and she changed a good moral man to a badly law abiding ethical guy.
#generalisation606 not all of them. some 'dreamy guys' actually have standards... some 'dreamy guys' are actually just as bad as those girls are, or some 'dreamy guys' have no idea what they want themselves.
it's just the same with good girls falling for bad boys. it isn't an always thing, but it can happen for a number or reasons.