This whole Mikey thing, I've seen, have split the MCR fans into seemingly a variety of camps, I, frankly, will admit am pretty much on Alicia's side, and I hope she will stay strong and not let this get to her even though it is pretty devastating for her. My mom is going through the same thing, and it's hell for her.
My opinion on this, is that, we do not know the whole story, and as far as I can guess from the tweets I've seen from Alicia, Lyn-Z and Gerard, I think there are a couple of possible sides to this whole situation:
1. Mikey was indeed cheating on Alicia, and very quickly got engaged to this Sarah girl while still married to Alicia.
2. Mikey and Alicia were already separated (or maybe divorced or not yet divorced), and Mikey just got engaged very soon after. (It's not like Mikey is going to announce to the world they're separating, it's their own privacy.) This would be pretty devastating for Alicia.
I know what Mikey did/is doing is pretty bad, hell, I am still really pissed off with my dad to the point that I am hoping I can develop a sense of apathy toward him, but people saying that they are now no long fans of the band is rather over-the-top. Because there are 4 members in the band, Mikey is only one individual, his action, no matter how awful it is, does not represent the whole band, so we should not just turn our backs on the band because of what one guy did. Fair enough, you can dislike him, hell, even I dislike him, but this does not mean I have to hate the band.
Some have argued that the band is supposed to be good role models for us, yes, that's true, as they have thousands of fans who really look up to them. A lot have also argued that it's none of our business, again, it's true, it's their own privacy, but the one thing that I could not stand reading a lot of people writing is that "People make mistakes"...so what? Yes, people do make mistakes, but if indeed situation 1 was what really happened, then let's me just say this..."People make mistakes" does not justify someone being unfaithful and having an affair!! "People make mistakes" is not an excuse for someone to go and do something fucked up, and some people just made it sounds like as if there is nothing wrong with cheating!! WTF??
However, no matter what, at the end of the day, we, the fans, the fandom, the MCRmy should not be split over this, it is not worth it, because it is the band and the music that we care about. And at a time like this, what we should be doing is supporting the band and those who were hurt by Mikey's action, especially Alicia (and I cannot even begin to imagine what are his parents feeling) not fighting. I know that with Mikey, it's going to take a lot of people a long time to get over this mess, and like I said before, you can still dislike him, but please do not spread you dislike/hate on to the other band members, they are after all, just innocent bystanders in this case, and I am sure that they are pretty confused/hurt/upset by what Mikey has done too.