It's a terrible trap to slip into but we say we cannot/won't change yet we're asking others to change.
Taken as a project of mutual respect we must recognize our significant other may even find the topic difficult to talk about.
I love the phrase 'significant other' and thank God that I have one.
Take any other difficult topic that's threatening in a different and less problematic way - say, buying a new car this year. Following a response from her that telegraphed she didn't want to talk about it any more soon, you might pick a moment to ask if you could bring it up again and if given permission, ask if she would permit you to open the discussion with her. There might be a few sincere 'I love you's in there -
The first thing to ask is for her to share why it's difficult for her to discuss. Say it in a non-threatening way and take the pressure off by saying there is no need for an immediate response, just 'after you have a chance to think about it, bring it up with me'.
In the typical man-woman differences there is the woman's need for security - that you are present for the long haul and you are strong enough and smart enough to handle your half of the relationship which means supporting and protecting her. She believes that your physical attraction to her keeps you with her and any hint that that is changing or fading or switching polarities in a way that she cannot or will not support means a threat to that security.
In the trenches of my relationship when I've found the things my wife really worries about I am touched, shocked, horrified at 99 percent of it. It is very hard for any of us to share our deepest fears even with that person we are closest to in life. But - if you can get her to verbalize even a single worry and then do your best to reassure her, your real intimacy (which includes but not defined by your physical intimacy) will grow in a way you both will enjoy.