Hi Bodymind,
I have looked into this as I was concerned for my husband once I realized that I had Lyme. As Opugirl said, not much monies have been devoted to this aspect of Lyme disease - yet.
Here is the only thing I've been able to find on the possibility of spreading it through sexual contact:
www.anapsid.org/lyme/bach.html So far it's only been found in, uhmmm, "male secretions" and breast milk and tears, and of course infected tissues, blood, and blood products as well. At this point there have been no actual studies, only what one or two LLMD/researchers have seen clinically.
So the best thing to do is to stay safe. If you are male (sorry), then I would suggest having your partner checked, or better yet, protect them. If you are female, the best thing you can do is to be careful.
What you can spread by kissing/oral contact is viruses though, so do be careful with that!
As for personal experiences - I've been married 3 times (chronic illness can be rough on relationships), and have had 2 children and not a single one of them has had any tick-borne infections. This does not say that it can't happen, but I've been "lucky" so far. Without proper studies done, no one knows the true risk that is involved.