What I mean by sexual attraction is looking at one person and knowing you want to have sex with them without questioning it. I feel that pretty rarely, but I still know who I want to date. I know plenty of straight people who feel attraction like me. I just notice people expect me to feel sexually attracted to girls and point them out, so I can prove myself.
No, I don't believe just anyone can decide "would fuck" or "wouldn't fuck" at a glance. For one thing, some of us are indecisive, yknow?
I'd say it's not universal: probably not everybody in the world experiences this. But for those that experience it, it is universal in terms of the same effect it has. I know that I experience this. I'm very lustful and ashamedly admit that I'm a big sucker for looks.
I don't think there's any one way that people experience attraction. Some people are more visually stimulated, others are attracted more by personality traits, smells, voices, mannerisms, whatever. Sometimes I can't even identify what makes a person I'm attracted to attractive.