Really? She doesn't ever like you to touch her butt? Are you talking public or private? I'm thinking you're talking both. Let's talk privately. First, if you two are intimate, you should be able to d/c this, and I hope you do. It seems quite normal to me that she not only let you touch her bum, but want you to touch it. This is, or at least can be a sign of affection. It's sexy, it's foreplay, well, you get my drift. But spanking is on a whole other level! When you d/c this it is definitely about sex/making love and it should be approached as this is all up to her. Of course, tell her that you would like her to give it a "shot" lol because it excites you and could really turn her on. And of course, have a safety word to make her feel she has more control in case you get carried away. Let her spank you first, give her a little control. You might like it!
Hope it helps.