You know that people who INSIST they are not gay are the most likely to BE gay? Especially when they insist that having a girlfriend means anything. You know how many gay people are married?
But its cool, we're not here to judge. Jay Johnson is right, plugging Is a great way to do opiates and you can rest assured that no one is going to know you're gay just because you tried it in private at home!
Just make sure you use plenty of lube ("Man Strokes" is a great brand) and be sure to slide the syringe deep into your love tunnel. Ideally you'll want to insert it, slowly pull it almost all the way out and insert again, just to make sure you are "prepped" (repeat this a few times before you release that hot juice deep inside you).
If you clench up due to nerves, stroking yourself as you insert the syringe is a great way to relax the sphincter. Maybe put on some music, like some Justin Timberlake for instance.
And remember, its not at all gay!