Best friend has become cold with me.....I suspect that's because of my being gay

[Bowyn Aerrow]

Time wounds all heels - and heals all wounds.

Eventually he will reach acceptance or he won't, eventually you will move on (if he continues to reject you).

[Ekwarph]

I agree with Bowyn, it's probably a matter of time.
I "understand" him (it doesn't mean that it's justified and fully forgivable^^).
You grow up together, when you don't know lots of gay people, it's normal to be a bit afraid of one of our best friends being gay.
Finally, he will understand. Don't be paranoid, cause when you anticipate reactions, you are more likely to put a distance and he would put distance because of you, not of your sexuality (I speak according to my proper experience).

Good luck. It will get better, I think.