people think its not normal

Because we know right from wrong.

Religion

Religion, insecurity, can't understand it, they are unhappy so others have to be unhappy as well.

Mainly religion and environment from what I've seen.

90% of them are closet case, and 10% are brainwashed by religious upbringing.

I narrow it down to three things.

Upbringing (Parents telling their children it's wrong)
Closet Case
Religion

Privilege. They see gays having love and success and they feel it must be wrong because it's not what they are.

Looks at Avi..

Basically this. Then they go and find different things to defend their stance (for example, religion) so they look less stupid and immature for acting homophobic and coming after other people for being gay

A lot of these people also subscribe to the "It's okay to be gay but don't do any of that gross gay stuff around me! Don't like shove it in my face!" unless they're using the "Homosexuality = Evil" defense

I live in one of the most homophobic countries in Europe.

It's mostly cuz of religion and people simply think it's not a normal thing.

It will sound weird, but even I was homophobic for some time Especially when I was aged 9 - 13.

And the word "gay" basically doesn't exist here. Everybody uses the f word

Because:

-Some bitter people just need a group to hate. It can be gays, immigrants, ethnic minorities, just about anyone that deviates from their own.
-Some people are just not educated on the topic and have been told not being straight is wrong and it's easier for them not to challenge that erroneous belief

Because they're arrogant.

This is actually a perfect example of one the main reasons: Stereotypes.

Because they low-key get off to a bit of anal or scissoring (depending on whether they're male or female) and are afraid to admit it.