Regarding your question as to marriage visa fraud, I don't know if France even has such Visas. If you mean could you claim one of these as a marriage to obtain a US visa, you would have to do it on your own or find a lawyer who didn't care about loosing his license for making a frivolous claim.

Hmm … residency permits are a deal in so many countries.
Getting married to your girlfriend to resolve all that hassle is common in this situation. Or to your friends. Like, where do you draw the line between this strict love contract of marriage and the practice of faciliating someones life by a great amount?

What I heard about the French model is that offering a second option makes these choices easier by asking fewer questions – you start out as sharing a flat, but now are somehow married; you aren't enganged or even in love but cement your ”old couple” status with a contract.
A lot of this boils down to acknowledging the many variants of “family” or “smallest society unit” or whatever you call it legally. And it's telling that this originated in a law intended for same-sex couples without giving them a traditional marriage option. ([Edit:] Ok, looked that up – traditional same-sex marriage was made legal some time later)

As I said, I know nothing about what France does but, having represented many in court on their visa denials ( as in, by the time they get to me they have been denied at the administrative level) I can say that something kinda like marriage will not qualify for a visa in the US. The laws are very specific in definition and moreover recognition of foreign marriage is based upon reciprocal treaties.