Do taken guys ever like other girls?

Yeah of course, but that is where commitment comes in to play. Yeah, there maybe a spark maybe even more but the difference between being single and being in a relationship (at least the way I see it) is not that once you commit to someone that you shut your eyes & heart to everyone else but that you agree to share certain parts of yourself with the one you have committed to.

it depends on the guy. and it also depends on his relationship with his girlfriend. and what he's looking for as well.

Yes. Gals do as well. Being with someone we like a lot can usually prevent a little flirtation from turning into something bigger, but not always. Especially if you spend a lot of time with the other girl (work, school, etc). Thinking that people lose their capacity to feel emotions for anyone but yourself once you're dating them can be a grave mistake.

Yes sometimes that happens when they are bored of their relationship or when they feel like they are losing it with the girl, or when they are "players"

all girls and guys look when there in a relationship but not necessarily try to be with someone else unless there a player but sometimes you can't help feelings and it goes over board.

It usually happens when you aren't having a need fulfilled in the main relationship. Go long enough without and you start looking elsewhere to fulfill that need.