@ryy...it pains me to see you agonizing over the thought...so let me try and help.
WHAT IS MEANT BY SEXUAL FANTASY?
There are many types of sexual fantasy. Sexual fantasies can be brief thoughts which enter your mind when you see someone to whom you find yourself attracted, or momentary sexual thoughts that might occur to you at any time.
What I believe you are struggling with is your more permanent, recurring sexual fantasies, perhaps those which you have experienced again and again over a longer period of time, as opposed to fleeting sexual thoughts. Longer-term fantasies are the private sexual thoughts or scenarios you might call to mind when having sex with others, or whilst masturbating. This type of fantasy will often be highly arousing, and often causes you to reach orgasm. These fantasies sometimes involve other people, specific places, and precise events.
Since you brought up S&M and light bondage, it is possible that you harbour deep seated fantasies about this form of sexual expression. As Annie Lennox said...
Sweet dreams are made of this
Who am I to disagree?
I travel the world
And the seven seas
Everybody's looking for something.
Some of them want to use you
Some of them want to get used by you
Some of them want to abuse you
Some of them want to be abused.
Only you can deal with who you really are behind the facade you present to the outside world