Let's say she is paying $3000 a month for her mortgage. He currently shares an apartment with two roommates for $500 a month. Why would he move in with her and pay 6 times as much UNLESS he could afford it and was going to get equity?

OTOH, if he was paying $800 a month but his share of her mortgage/taxes/insurance would only be $500, it might make sense for him financially to simply pay her rent. HOWEVER, if he wanted to marry her, insisting on being on the mortgage is a way to get clarity.

It's not about fairness. It's about determining if they are on the same page.

As I made clear in my article - if neither one wants to get married, it DOES NOT MATTER.

But if one person wants to get married, he or she will want their partner to act like someone who is planning to get married. Treating the house as something they both own is a signal that both parties are serious about marriage.

Of course, this is my opinion and I obviously disagree with you. And I am fine with that.