This does not mean that it was sexually transmitted. You can get it a couple of different ways and just because you have it does not mean you pass it.
Your partner either had a false negative, you had a fasle positive (both rare), partner got it, gave it to you, but took medication to clear it up or you got it and never passed it on.
This is not an STD... here is a link for more info.
http://emedicine.medscape.com/article/231470-overview
Also here is an Experts response about ureaplasma
http://www.medhelp.org/posts/STDs/Ureaplasma/show/245287
Let us know if you have any other questions