Why do some people believe that gays are born gays?

Because they are born that way hun.

I chose to like vagina. It’s called, hey ladies are cute, hey I like men as friends. Boom off that I chose to be straight 

Observation. The basic personality of an individual sure does appear to exist at birth, and I know several examples of people who were obviously gay even at a very young age and who were essentially always the way they are.  It follows from logic that  were born that way (not a learned trait).  This does not apply to all, but it definitely applies to a significant proportion.

Well not everyone is the same that's just a generalisation 

Because many scientists believe that they are.

There are enough small differences in development of certain parts of the brain between homosexual and heterosexual that there is evidence to support that they are born that way.  Some studies think that it has to do with when the fetus is exposed to certain hormone levels during development since they have been able to make homosexual rats by putting sex hormones in the mother at critical times.

Nothing is conclusive, but there is still ample evidence to support this.

There is no evidence to support that they were not born that way.

Because they are. What a stupid question. 

The same reason why you believe that heterosexuals are born heterosexuals...?

for the same reason that you were born stupid.

They are born that way.

Good question. It usually doesn't begin to show until age 5.