Mate I have a heap of Business finance, and I am guarantor on every contract (I am also the sole Director). They simply want to be able to make claim on your personal assets should it all go pear shaped - it also gives them a feeling that you are committed to the Business.
After all, if you won't guarantee it, does that help them form the perception that what you are proposing is something you believe in? If you believe in it, guarantee it. If you don't, then don't get in in the first place!
And don't try shifting all the assets into the wife's name - they'll smell a rat, and you'll find it harder to get finance anyway.
Cheers,
Tim