Her partner is A-san(38) who is the CEO of an IT company who looks like the actor Uchino Masaaki (43). He provides software for one of the cable networks and has been staying at Nakazawa Yuko apartment.
They were introduced to each other by a friend and started dating from August this year. In October a photo of the couple was released in Friday magazine. According to someone close to the couple recently A-san proposed to Nakazawa and after only 4months it is a "speed engagement".
The plan is once Morning Musume OG group Dream Morning Musume finishes with their performance at Tokyo Budoukan on the 10th of March they will have introduced each other to their parents and are aiming to have a late Spring to Summer wedding. Then they will begin their married life together in the groom's city Fukuoka.
Nakazawa is planning on moving between Fukuoka and Tokyo for work.-Source (sanspo)
Sorry for the very rough summary of all the important points in this article. It's not word for word.
I'm kind of disappointed she didn't marry Sakamoto. They were cute together! It's going to be hard going back and forth between Fukuoka and Tokyo though I imagine.