I had no idea she had a boyfriend, let alone one for a while now.
The rumors about Aya and Tachibana have been going on for years. Officially, the management groups of both sides have almost always been "No they're not." Aya and Tachibana themselves however, would usually just dance around the issue or just be "no comment" if the issue was ever brought up in interviews. With all of the rumoured sightings of them though (and this is an Aya fanboy saying this), it's really always been pretty obvious that they really were and have been a couple all of this time.
Now considering how the Idol industry (and to a certain extent, the Entertainment industry in Japan in general) works, to me it would not be a surprise at all if the current rumors were true and that she really is planning on retiring from public life completely so that/if she wants to settle down. Many an idol in Japan has followed this route before Aya. Now speaking on a personal level, I still hope that if she and Tachibana do decide to settle down/get married/whatever, that she still stays active to some degree. Sure she pretty much stepped back/walked away from the singing part of her career, but that's okay. For me, Aya does not (and for a long time she did not)
need to be "Ayaya". I've been totally fine seeing her "just co-hosting" Meringue. She might not have been the headliner of the show, but then again, she was never supposed to be, and as far as I could tell at least, she was fine with it.
We've seen that it's possible for idols to start more "grown-up/family" oriented phases of their lives (from H!P alone, there's Miki-sama, Nono, Kaorin & Ayaka) and still stay in the public eye, even if it's on a reduced scale. We've also seen examples of idols who have previously walked away, then later on decided to re-enter public life (such as former MM member Icchi Sayaka and former C-ute member Murakami Megumi), so even if Aya decided at this point of her life to retire, that doesn't necessarily mean that she wouldn't entertain the possibility of returning sometime in the future. Then again, who knows, she might find something (career-wise) that she is just as good at, or possibly even better at, than when she was being an idol/singer.
In the grand scheme of things, we as fans need to realize and accept the fact that Aya will do whatever it is that makes her happy. Whatever she wants to do, is what she should do, and the best thing we can do is just wish her well/wish the best for her and hope that she does indeed get that happiness in life.