You are not sinning if you are truly attracted to your spouse to be. I think it is wonderful that you and your fiance are devoted to remaining chaste (one is still chaste after marriage if you only have one sexual partner–your spouse!)
I have used NFP since we were married 9 years ago, and I can honestly say that our physical relationship has only gotten BETTER with time and three kids. In fact, the times our children were conceived–and we knew that was likely at the time–are among our, erm, “best” memories of physical intimacy.
The Church does not teach that a couple may not enjoy the physical act of sex. God designed us to enjoy physical intimacy, and so it is not sinful to enjoy sexual intimacy within a marriage.
As for “only allowed if you are trying for children”, that is a misinterpretation, I think, of the teachings of the Church regarding artificial contraceptives. If a couple is using NFP and is aware of the times when conception is least likely (and I’ll admit I’m blessed in this regard, I have a very regular cycle and know exactly when those times are), they are not forbidden from enjoying physical intimacy when conception is unlikely, if they are refraining from having children for sound reasons. Those might be financial, emotional, or physical.
In my case, I have a rare autoimmune disorder that causes my platelet count to plummet when I am pregnant. This puts me at a serious risk for bleeding out during delivery (I almost did with my first child). Because of this, we have decided, after three children, to refrain from actively conceiving for now. This is a sound reason, and we have discussed it with our priest. We do not use artificial birth control, but restrict serious intimacy for when my least-fertile times occur.
There are many ways of being physically affectionate without being sexually intimate. And as you get older, and more tired you begin to appreciate those! Sex is just one component of a loving marriage, albeit an important one.
For my part, I really like the practice of NFP. My husband has always been on board with the periodic abstinence, which assures me that he didn’t “just marry me for that”, which some of my friends have pondered regarding their husbands.
Anyway, you are not sinning when you find your fiance attractive. It’s a blessing, in my opinion, to be physically attracted to the person you love. Although yes, it does make waiting for the wedding night harder, I suppose! would pray to Our Blessed Mother to help you in your resolve. Saint Maria Goretti is also a good saint to pray for, to help with remaining chaste.