First off, let me make something clear-- no act is a gay act if it's being done by heterosexuals.
Gay people breathe. This doesn't make breathing gay. Gay people eat cheeseburgers and drive cars and mow the lawn. That doesn't make John Deere, Johnny Rockets, and Chrysler bastions of homosexuality.
Yes, having your prostate milked might not be something you've come in contact with before, but if you're open to trying things, it feels amazing!!
Before you do this, go to the bathroom and try to defecate. Wipe very well, or take a shower afterwards to make sure you're as clean as you can be.
What was it like? First, you have to get over the fact that something is in your ass. No, this isn't something you normally experience. YES, it will feel weird and your body will be sending signals to your brain like, "Dude, WTF??! WTF is going on? Dude? DUDE!"
GET OVER IT. It's just a finger for god's sakes (or two, if you're daring and she's wicked).
And besides that, it's what the finger is DOING which is more important. Have her put lots of lubricant on her finger and around your rectum so she can slide easily in and out.
She's going to slide it in as far as she can go and then curl her fingers like she's making a hook and rub firmly on the flesh underneath your scrotum through the inside of you. At the same time, she should either be giving you a "Hand" or "blowing your mind." You're going to feel this massive shivery rush and you're going to orgasm much more quickly and possibly much harder than you normally do.
That's it. No, really. That's the earth-shattering life-is-coming-to-an-end whole thing. Your girlfriend wants to make you orgasm really hard. And because you're doing it with her, and you think she's hot when she's making you orgasm, that makes this a very heterosexual activity, as much as when you give her anal. If it you and her, no matter what you do, it's not gay because you two aren't gay.