Sorry for the late reply to a post but here's my take, coming from someone who was raised in a Catholic house (even though I'm no longer Catholic, I still consider myself a Christian). I really don't want to sound like I'm preaching but the answer is yes it's sinful, but the whole point is that we are inherently sinful people and that no matter what God forgives us if we let him into our lives.

My advice is have 3 conversations. One is with yourself, one with God, and lastly one with the person you'd probably experiment with. The conversation with yourself should be about what do you feel like you need in your life, who you are and who you want to be. Something my Ex told me when we stopped trying things in the bedroom was she thought that sexual fantasies stem from something you feel you're missing in your day to day life. I can't really say much on what you should talk to God about, there's a whole boatload of philosophical arguments and it depends on individual beliefs. Last, if you have someone in your life who can be considered a sexual partner you have to talk to them about this and see if they feel comfortable with the idea as well.