Who benefits more out of a marriage (and why - optional)?

It can be mutually beneficial for both parties. And that should be the goal. This is what makes a good marriage, a good union, a good partnership. Anyone who says one or the other will likely be discussing a personal bias and is far too fond of blanket statements.

The one who is less financially and/or socially well off. If an unemployed person married a billionaire, regardless of gender, who benefits more?

I don't see a point in marriage at all. If anything, it shows more insecurity than commitment.

Women, mostly. Ofcourse, there are many that are mutually beneficial to the same amount but on a basis where one partner benefits more, it would be women.

Given that women make the majority of the spending in marriage and in general. Given that most of the time the male is the one working (not all the time, don't bite my head off), and given the risk of divorces and the wayyyyy unfair divorce settlements that tend to favor women, and given most divorces are initiated by women, I would say women.

And I've also heard and believe seen a study stating women are less sexual after marriage and that couples not married have more of a sex life.

So, overall, I would probably say women.

Both if they're happy together. Silly.

Because in Western society woman are dominant and treat men like shit.