IMO in order to use MM's name, title and status to make himself more credible and part even more, and potentially richer people from their money.
And to access even more potential ... shall we call them clients.
- And of course because they are so much in love...
I do in fact think they genuinely enjoy each other's company, however I also believe this marriage is more one of convenience - benefiting both of them.
No, he has been married to a woman, Zaneta Marzalkova, for a few years (2005-2009) but reported her to the immigration authorities in 2008 and she was deported. Afterwards he was in a relationship with a man whom he was engaged to and supposed to marry in 2015 but they broke off before the wedding. He also lived together for 6 years with another woman - Tiana Griego - (it is unclear to me where this fits in the time line - and while it seems it wasn't a romantic relationship, their relationship seems concerning). Accoording to Tiana "Durek controlled my whole life. It was as if he became jealous of anything that stole his attention. I was not allowed to start a serious romantic relationship or raise my son. It was all about Durek." (see background on his wikipedia page for the original sources)
Verrett has a sketchy background and is rather indiscreet but I can honestly say that I don't get malevolent vibes from him.
He talks too much, and let's just say he is into hyperbole to put it kindly.
But Martha Louise is no snow maiden. She is a middle aged woman with enough experience outside the Royal cocoon to know what she is getting into. And if she doesn't, it's not her finances fault.
For the sake of her three vulnerable young daughters I hope everything works out well.
They are vulnerable because they were semi-orphaned in traumatic circumstances. They are very young.
They might be very open to having a father figure...one who fills their minds and imaginations with all sorts of crazy ideas.
There was one idea in particular about sexually active women having an "imprint" that needs to be purified.
I won't go into more detail here. Look it up.
www.vg.no
While King Harald V has been an outstanding monarch in many respects, I am sorry to say that his management of constitutional issues is one area where he has been wanting. A constitutional act of government which authorizes two people to remain or become eligible to serve as the country's head of state and first gentleman should have been handled with far more clarity, transparency and formality than the court press officer sending an email to news outlets telling them how to interpret the Constitution based on the King's courtesy messages to his family members. 
I suspect the establishment and most of the public in Norway will accept it as such. The ambivalent wording is likely reflective of the ambivalent public opinion. According to what Royal Norway wrote here, it appears the general public detests the idea of having Princess Märtha Louise and Durek Verrett as their Queen and Prince consort - but at the same time, they detest the idea of removing Princess Märtha Louise and Durek Verrett's legal right to become Queen and Prince consort when they marry.
