Just because it's occasionally found to be in sexual secretions and even between partners doesn't mean that this is an efficient mechanism of infection. Pretty much all bacteria can be opportunistic pathogens if host factors allow. All this means is a bacteria is exploiting an available niche at the expense of a host. A niche which may not be available in every, or even most, hosts. However, Borrelias long standing tendency is to spread from ticks and therefor it has refined it's strategies towards this method. This is its natural cycle of transmission. This is how it avoids extinction and this is where selective pressures act most strongly. Everything else is chance as far as we know and doesn't contribute as much to its survival.
Now I'm not saying sexual transmission doesn't happen. Or that it doesn't happen frequently. I don't know what the probability is really. Just answering the question of why some couples share it and some don't. Most pathogens don't transmit with 100% success even in their natural routes. The success drops off even further when other routes are involved. Any infection by these routes is better thought of as a matter opportunism. Same reason some ladies are prone to vaginitis. Most of the culprits are very very very common, like Candida, but only cause problems when host factors such as lack of competitive microorganisms, genetic susceptibilities, and other abiotic factors are involved. I still suggest protection because even a 1% chance is more than I'm willing to gamble with in my partner over the long haul
Post Edited (Psilociraptor) : 2/20/2018 7:57:25 AM (GMT-7)