Could I suggest reading MILKING A MAN'S COCK thread first. If you're looking for a prostate O you'' need to "learn" the feeling first. The MILKING thread gives good instructions and guidance and common sense for doing this.
Getting fucked in the ass by your wife is probably NOT the way to get into this the first time. IMHO.
Why are you nervous? I obviously don't have a prostate, but I understand that prostate stimulation can cause some really intense orgasms. And *psst* liking anal stimulation doesn't mean you're gay, or even bi. It just means that you like having your ass played with.
Perhaps your wife should start with well-lubed fingers and work her way up to the strap-on over a series of sessions. I would have probably been a little nervous if my husband had just tried to plunge his dick into my ass when it had never had so much as a finger inside it.
For what it's worth, consider yourself lucky that you have a wife who's open to trying something like this. Some men on here would give their left nut (well, maybe their right arm ) for such adventurousness.