Yeah they can, you can find your opposite sex friends attractive without it being a barrier to friendship.
What is a barrier is if one party fancies the other to the extent that it causes feelings of jealousy, bitterness etc, and the feelings aren't returned. In that case its usually not a friendship its either that party hanging around secretly hoping for more (when it won't happen) or hanging around having accepted nothing will happen, but still doing it anyway because they have become dependent on the contact from that person.
I think its a real issue for some guys who are just not very good at forming real relationships, when they get a close female friend then effectively that friend fills that gap in their life, ok they aren't having sex but they are getting a girl to talk to a lot and to share their feelings with and then if that girl gets a boyfriend it will feel like a break up to the guy.
These kinds of 'friendships' aren't healthy and will cause problems.
However in general opposite sex friendships are healthy and good and I have much more meaningful chats with my female friends than male friends and find I can talk to them more naturally about some things when I'd feel awkward with other guys. I find most of them attractive and if they are dressed sexily or whatever I notice but it doesn't change things.
There are also opposite sex friendships that have the potential for more, and sometimes this develops, sometimes it doesn't, it just depends on circumstance. For there to be the potential for more there needs to have been some mutual chemistry at the start, and this can happen when one or both partners are in a relationship with someone else. At a later point circumstances can change.
But I think there are some friendships that always simmer around at a level higher than usual friendship, there's an underlying tension there. Sometimes these can be volatile and have a lot of arguments mixed it with a lot of times when you get on incredibly well together. Again whether you get together will come down to a lot of other factors but you can always tell it's not a normal friendship.