This is something I have been thinking a lot about recently. I'm aro-ace so personally, I get neither one side nor the other. I've seen some stories (e.g. I think there was one in our newspaper a while ago?) where people share how they met though and there's been a fair share of people who saw somebody and went 'That's the person I want to marry!' and then they did. I'd say that probably counts as love at first sight? So I'd say, yeah, it's possible. Just not as likely as Hollywood would like us to believe 
This is also something I've been thinking about and I think for me the answer is somewhere between 'There's different kinds of love' and 'We all make assumptions'.
Like, let's not talk about relationships for a moment. Have you ever met a person and thought 'Wow, I wanna be their friend'? This has actually happened to me. And it's less about how they look and more about the impression they leave me with.
I think that there are just some people around that seem so pleasant even at first sight that you feel instantly drawn to them. This might have to do with how they look as well but you don't have to be a beauty for that. Even a person that isn't conventionally attractive can seem very pleasant to be with.
Now, I think that when we talk about friendship, this is easier for people to agree with. As soon as it is about romantic/sexual relationships, people will start to find it shallow. But I think (maybe that is because of my orientation though) that it's basically the same thing. It's not just looks. It's the impression they leave you with which has to do with looks, behavior (e.g. the way they stand/move/sit/use their hands/laugh/smile), maybe even the tone of their voice or what they're doing at that moment.
So I think it's not necessarily shallow or superficial or anything and doesn't say much about us other than 'I like this type of person'. Now, for some people, that might not count as 'love' already but then again, I think that people can also fall out of love later on if they notice this person isn't what they originally thought them to be.
At that stage, it's an initial feeling of romantic attraction - maybe contrary to one of sexual attraction since we're making a distinction here.