This app was mentioned in 4 comments, with an average of 2.50 upvotes
Seeing as the site appears to have been hugged to death, I'm hijacking this comment to plug an Android app I worked on that does the same thing. (It has an iOS version too.)
I'm not being paid to push it, I was just the primary developer on the Android version and I'd like for us to get some more useful feedback on it. We're also getting ready to roll out a big update with a bunch of new features.
I would actually recommend a mobile app for you and your girlfriend, especially if talking about each other's sexual needs feels uncomfortable or if you're not sure how to broach the subject without leaving one or both of you feeling hurt.
My boyfriend brought it up to help with the discussion of asexual turn-ons, turn-offs, fantasies, and potential fetishes. It's called kindu for couples and can be downloaded here - Android and iOS. Each of you download it on a mobile device and go through lists of sexual acts and ideas. You rate whether you really want to try it, are possibly willing to try it, not sure, or are not willing to try it. Then once you've both rates some different activities, the app matches up activities that you were both willing to try. IIRC, it doesn't show your partner your actual ratings for each item, so you can choose to show them each rating or not, if you're too shy. Also, as a side note, I don't agree with the responses saying "just do it for her" - she will be able to tell if you are not enjoying it at all, and you should be able to find some turn-ons that you discover you both enjoy.
Nothing can replace open discussion with each other, but it can be a good place to start. There were many items on the list that I found my boyfriend and I were both into but too shy to bring up (after over five years together!). It gives your girlfriend the opportunity to get a feel for your boundaries and what you're comfortable with, without just hearing an explicit "I'm not into what you're into sexually" (which can be highly discouraging). You may even find turn-ons that you've never even thought of, and that she likes as well.
I think the most important thing is that you're willing to undergo sexual exploration with her and are willing to try everything (obviously with a few exceptions) at least once so you can find new things to do together that you both enjoy.
Sex is like everything else in life - you NEED variety. It's just like you have a little variety in your day-to-day activities such as going to dinner, a movie, a zoo or museum, etc - you wouldn't be happy living the rest of your life just going to work, coming home, eating dinner, and going to bed. Likewise, you need to bring some variety into your sex love occasionally and do something different - most people wouldn't be happy only doing the missionary position quietly and lovingly every single time they have sex for the rest of their lives. I don't agree with the people saying "just do it for her"; she'll be able to tell you're not enjoying it.
Personal Experience:
My boyfriend (27M) and I (23F) have been together for six years now. The kindu app helped a lot with opening up discussion between us, even if it still has to be via text sometimes when we feel shy saying certain things. We are now more open about bringing up sexual desires, and will often delve into why we think we like our dislike certain things. There are a lot of fantasies that we both have that we don't get to take part in, but we still have a great sex life.
I am into BDSM and he's not, but he will playfully slap my ass, pinch my nipples, bite my lip, take part in control play (eg you can't cum until I say you can) even though he's not very good at it (I love that he puts in the effort to try), and has even enjoyed using restraints (velvet hand and ankle cuffs at the four corners of the bed - he even had me restrain him after he saw how much I enjoyed it!).
Likewise, I've tried (after he's told me that it would never freak him out or upset him) getting him hard then taking his boxers off while he sleeps then waking him up to me on top of him or blowing him, a foursome with lesbian play, being "mouth-fucked", doing webcam shows, massaging his prostate, ass-play, dressing up in risqué lingerie, talking dirty, being okay with letting go and squirting even though it's messy, surprising him when he gets home from work after me, and bringing toys into the bedroom.
The most important and beneficial thing for us had been keeping ourselves open to trying new things and, even more so, keeping ourselves honest with each other about what we like and don't like as well as why we feel that way.