Hello Becks,
A man's view here...
What a lovely and understanding wife and partner you sound like. And DH sounds lovely too.
I don't think there is anything wrong with how you are feeling. Variety is the spice of life and sexuality is such a broad spectrum that it is, IMHO, nigh on impossible for one man to satisfy all of your needs/fantasies - ALTHOUGH I AM NOT SAYING HAVE AN AFFAIR!!
Obviously that leaves two choices - either DH gets into it with you, but you seem pretty certain that it would not be his thing, and even if it was you might piss yourself laughing. Or, just leave it as a fantasy. There is nothing wrong with fantasy and there is nothing wrong, IMHO, with occasionally acting out that fantasy in one's head while making love to the person we love.
Alternatively, you can act it out on your own, maybe even with the help of some porn - although I say that with hesitation, in view of my porn thread which has just hit 500 posts! (Please do not turn this into a porn thread, there is another one nearby!!!).
And as I have often argued elsewhere in this parish, having a secret garden is, IMO, not a bad thing. One person cannot meet all our needs all the time - and I say that as someone who has never been unfaithful in actions or in real life but who does sometimes use porn and my own imagination...
As for why do you have this fantasy, well it would need a more skilled person than me to know. Maybe though it is something as simple as we often want what we cannot have. In truth, I sometimes think about previous lovers, some of the things they did which DW would not do. It's not a question of being a better or worse lover, it's just a question of being a different lover.
Hope that helps...