This is one of two things, either she's not attracted enough, or she doesn't trust you enough. The third possibility is that she simply has no desire to do that act, in which case there's really not much you can do. Likewise I know I have certain things that I will never do to a girl no matter who she is/how much I like her, and I'm sure you're the same. Accept it or find a different chick if its THAT important to you, you won't be able to change her.
Seeing as she said she'd only do it with her husband instead of "No way am I ever doing that" (which btw I've often found to be inaccurate anyway...), I think that basically tells you its a trust issue.
So for example if you want to make a video of you doing her in the ass, well that's usually not something girls want you to go off telling your buddies about.
What worked for me was just having an agreement and actually honoring it that we don't discuss details about what we do in the bedroom with other people. Sure, you miss out on the reputation boost you might get if she's telling people about how great you are, but in my opinion the increased willingness on the girls part to 'get freaky' ifyou will more than makes up for it.
So my advice would be to not force it, adopt that dynamic over time, and try again later.
PS: Its really hard to give you advice as you have given us no info - this situation would have to be handled differently if she's a one night stand compared to a LTR for example.