It's not always easy to get the words out "Honey, I'd like you to strap on a cock and fuck my ass."   For some men, it's downright terrifying.  Somewhere along the way we decided that assplay was dirty and closely tied with being a homosexual.  For that reason many fear their partners making assumptions and looking upon them with disgust.  So I always recommend a little education in those situations.  These two videos are fantastic to watch with your partner, whether it be for him or her.  Don't judge the videos by their covers they are extremely educational, funny and downright sexy, with several pegging sessions between different partners after the "how to" part.

Expert Guide to Pegging
Expert Guide to Anal Pleasure for Men

Now, if your partner is sexually open and up for most things, here are a couple of ways to gradually expose her to the idea of pegging: 

1. Start A Couples Blog on Tumblr - This is a great way to explore as a couple all of the things that turn you both on.  You would each be able to post to the same blog by reblogging pics you see that turn you on.  It opens up dialogue and is easier than just blurting out what you want.  You can start with light assplay pics and scenarios so he/she gets use to the idea that it turns you on to have your ass played with.  Every couple should start with some basic play before jumping into pegging.  Fingers, small toys, rimming, etc.  Once she is open to the idea that your ass is a pleasure zone, the fun can begin!

2. Watch Porn Together - Watch some porn together that has elements of assplay (fingering, rimming, etc).  Really let your arousal be known when those scenes come on so your partner gets the idea that this is a turn on for you.  If it doesn't open up dialogue right away then the next time you are in bed and both aroused, mention how much that porn turned you on the other day.  Ask her if she would be into giving it a try.  Tell her you've heard how pleasurable a man's prostate can be and you'd love to try it.  If you make it about your pleasure then chances are she will consider it.

3. Bring It Up During Sex - If you aren’t into watching porn together another way to open up conversation is to bring it up during sex.  If you are comfortable with it, let her know how much you love when her fingers brush your taint when has her fingers down there…or better yet her mouth.  If she is sucking or licking your balls, you can tilt your hips up and when her tongue slides down be verbal (moaning) and let her know there is a new pleasure zone she may be neglecting.  Same with fingers, if she is rubbing your taint, work your hips so that her fingers inevitable graze your asshole.  Again be verbal, like she just did the best thing in the world.  If you are feeling a bit forward and know she is in her kinky zone, grab her fingers and drag them lower, working them against your hole.  Tell her how fucking good that feels.  Once she is comfortable with playing with your ass from the outside, encourage her to work a finger in.  Tell her you’ve heard how hard guys can cum with their prostates stimulated and want to give it a try.  Once she is comfortable fingering or rimming your ass…then you suggest a toy (perhaps in your case not cock shaped right away).  You want this to be about you and her and exploring new pleasure and not about her being dom or having to be the "man" in bed.  Even if you crave your partner to be dominant, there will be time to ease her into that later.  Baby steps!

4. Phone Apps - For a more direct approach plan a hot night where the theme is reveal secret kinks to each other.  If you have a smart phone download an app called, Kindu (available for Android and iPhone).  It’s basically a series of questions about kinks and scenarios and you each answer them separately with things like yes, no, open to discussion.  I recommend going down on each other while the other one answers the questions!  Then you review them together and see what each others kinks are, which ones matched up and which ones didn’t. At the very least she will read through all your answers and see that your ass is turn on for you.