... Is this a strange feeling/fantasy, or what would you call it?
I would call it the exact same thing that I experienced, after what seems like a lifetime of happily married heterosexual experience.
This concept comes up often, in many different contexts. The responses here, and to all similar threads, are sincere and heartfelt yet vary widely depending on the backgrounds of the responders. I blame most of the disagreement and confusion between us on the use of labels and stereotypes, yet it's inevitable considering the society in which we live.
It takes a very open mind to go beyond our society's social mores and puritanical constructs to consider experiencing that which you are currently contemplating. But if you can pull it off I am sure you will find the experience very rewarding. The most important thing for you to do is making absolutely certain that both you and the big-dicked guy that you hook up with are totally comfortable with the whole situation. Beyond that I would advise you to put all thoughts about how "gay" that makes you out of your mind, just enjoy the moment.
Like you, I originally listed myself on this site as 99% straight/1% gay. But by participating in these forums I was questioned repeatedly about that ratio, due to the fact that I am an inveterate cock worshipper. So I've changed my ratio to appear more bisexual. When it comes to having a romantic relationship or "making love" to another human being I am definitely 100% straight. But when it comes to just sharing good clean hedonistic sexual pleasure I am definitely bisexual. "Straight Lover of Cock" would be the perfect label for me, but there are too many out there that say if you lust for cock or (heaven forbid!) ever touch another guy's cock you can't be straight. I disagree with that viewpoint but, as they say, different strokes for different folks.
Good luck in your quest.