
From my very basic experience (which was the research & risk impact of world wide diseases/viruses etc), I would say that the person living with the dog should exercise caution and have a very very good hygiene regime, because primarily the lepto virus is transmitted via urine and other bodily fluids (as well as via cuts, abrasions on the human body coming into contact with those fluids).
You are on the right track with your suggested 'mode' of transmission, namely sexual, congenital or neonatal.
However, human to human transmission is considered rare.
Have a little look at this info here from a World Health Organisation publication and it might help shed some light.
Of particular interest is page 33.
http://whqlibdoc.who.int/hq/2003/WHO_CDS_CSR_EPH_2002.23.pdf