Maybe they're only attractive to you BECAUSE they have boyfriends. You see that they are desirable to men and therefore you want them too...
Also, you let yourself think of them that way.
When I know a guy has a girlfriend I would never ever think of them romantically/sexually. Like, they just go straight in the friendzone once I know and I would never think of them of anything more. You're obviously not doing that and still allowing yourself to admire them and think about them, they're unavailable to stop wasting your time :P
Probably because you are relaxed around them, since there isn't any pressure to try to impress them.. and of course, no pressure on them either, so you can have a stimulating conversation without holding back and weighing your words.
You can get to know them MORE EASILY than if they were single, in many cases!
So it's not rare to fall for them... boyfriend or not!
Cuz I can guess u want pretty popular girls!! If u looked for this kind u'll find a Sold Out! sign nearly where ever u searched!! My only advice to u: look 4 someone smart, cute, & not too popular in ur School/Work place. Gud luck!
Hard to say, maybe they're more attractive to you because another man values them enough to have a relationship with them. Maybe the girls act differently when they're in relationships, and you find their behavior attractive.