A baby can understand nothing. The instincts are triggered at a much later age.

Read the entire sentence.

"provides little theoretical insight into the aetiology of homophobia"

It doesn't delve into its causes.

So you're saying when it understand the concept of a relationship, then it hates homosexuality since it's not the norm.

But that's not genetic. That's the environment saying that heterosexuality is normal and the child thinking that homosexuality is not, it's not genetic.

That line you quoted is refereing the previous research, in contrast with this particular one.

The aim of this study was exactly that, to determine the etiology of homophobia.

-it won't be when its common and this is helping it become common

You need to learn the basics in genetics vs environment and how they co-op. Very often a human behaviour has a genetic basis that needs to be triggered by a proper environmental stimuli.

Read the small text I provided about the monkey fear of snakes for a quick understanding of the mechanism (Ill bold the key part of it):

"The classic and best experiment in this is Susan Mineka's work with a group of monkeys in Madison in the '80s, where she set out to examine the ontogeny of an instinct—in this care fear of snakes. Wild-born monkeys are afraid of snakes. They're so scared of snakes that they will cower in the back of the cage screaming rather than reach across a plastic model snake to get at a peanut when they're very hungry. Captive-born monkeys are not afraid of snakes; they happily reach across the model snake to get at a peanut. So what's going on here? That means that fear of snakes must be learned. But how on earth do you learn fear of snakes? The conventional classical conditioning wouldn't work very well, would it, because either you have a bad experience with a snake to learn from, in which case you're dead, or you don't have a bad experience, in which case you don't learn that snakes are frightening. So how are you going to end up acquiring a fear of snakes? It seems an absurd thing to acquire. She argues that what's happening is that there is a program for fear of snakes, an instinct if you like, but that that instinct needs to be socially triggered—in some sense triggered by a vicarious experience, by observing another monkey having a fear of snakes. So she set up an experiment in which she videotaped the wild-born monkey reacting with fear to a snake, and she then showed this video to a captive-born monkey, which immediately acquired a fear of snakes and was not then prepared to reach across even a model snake to get a peanut. She now doctors the video, so that it has the same monkey reacting in the same way in the background, but the bottom half of the screen now instead of having a snake has a flower. Again, the captive-born monkey has never seen a flower, so after it sees a monkey reacting with extreme fear to this new thing called a flower it should just as easily learn a fear of flowers. But it doesn't. It just learns that some monkeys are crazy. So what's going on here is that there is clearly an instinct for fear of snakes, and that's not surprising. Human beings have snake phobia. It's the commonest of all the phobias, even though most of us hardly even ever see a snake in our lives, but it requires an input from the environment. It requires a nurture input to be triggered. We know this is happening in the amygdala, and we're getting a bit of a handle on which cells are involved. We're not yet down to the gene level, but I'd bet my bottom dollar there's going to be a little pathway of genes in here that's mediating this process."

http://www.edge.org/3rd_culture/ridley03/ridley_index.html

Ah, OK.

There will always be repulsion to anything not normative though. I do doubt that that is genetic though. Children's beliefs of what is normal are instilled by everything around them. They think that what is not normal is "weird". It's not wired into their genetic info though. It's just the nature of their environment. If someone was taught everything was normal, they wouldn't be repulsed at certain acts, would they?

Instincts are adaptations known from birth to help an organism survive.

So... how is homophobia supposed to help one survive?

Adaptation would not allow a simple repulsion to become instinctual since it's not necessary for survival.

It's indeed helping the organism to survive. You put a repulsion there and voilá, you won't have sexually aggressive males wasting time on courting other males who won't give them offspring anyway.

That's a stretch if I ever saw one.

This, of course, also needs the idea that sexuality isn't genetic. If heterosexual males (and let's say they're born with that orientation) didn't need to worry about being in love with other males, why would they need the repulsion if they would still procreate without it?

Although, if you say they're not born that way, my point is moot.