Can you just assume a more macho persona? Walk with a more masculine gait, speak with a deeper voice? Mask any effeminate traits through physical control. I know it's unnatural to behave other than what comes naturally but you asked and this seems like the quickest fix and is a drug-free option.

No...don't do it. Drugs can really mess you up - although you didn't say which drugs you were looking at taking (I assume steroids). Other drugs may do more harm, since you might not have the faculties to handle situations that arise. I suggest you just act a little crazy...talk to yourself, yell at imaginary people, bark at your socks...just do odd shit and people will typically leave you alone b/c they don't know what kind of crazy shit you will do next.

Inject Testosterone and watch lots of John Wayne movies:

How did they know you were Atheist, and how do you know that is the reason you were denied?

Just be yourself

If you are acting, as you typed, stop. If it's you that is showing, accept and embrace it.

It's sad that you can't just be yourself. If you do get into General pupulation, carry a gun on your hip and wear a body cam so that if anyone threatens you you can protect yourself and have evidence of self defense

I'm sorry that you have to worry about that.
We are here for you.