Should is a four... er... six letter word
As a quick refresher, structural functionalism is a sociological view that holds that everything that occurs within a society has some kind of function. Some functions may be openly stated, some functions may happen unintentionally, but there are functions. There's also an evolutionary undercurrent here as well - an implication that any institution, group, or function that persists must be a net positive for society. Otherwise, it claims, that part of society would not be able to continue. 
That last part frequently leads to a judgment call - that therefore, whatever exists now in society should exist, and must therefore be useful and good. And that's crap.
However, this viewpoint has an emotional appeal to many people - because it's easy. It means that whatever currently exists should exist, exactly the way it is now.  Just because it gets by the way it is does NOT imply that it cannot be better.
Look at marriage as one convenient example. We're still hearing a lot about the "traditional" view of marriage  being under attack. Divorce rates, same-sex marriages, and people wanting to marry toasters oh my! Okay, so the last is a little nonsensical, but the other two aren't as big of a deal as they're made out to be.
Or at least, they don't have to be.
The functionalist viewpoint tends to avoid splitting off the function from the activity itself. For a practical example here, it holds that marriage is there to raise children. Which is a function - but not a function that requires any particular kind of formal relationship between any particular gender of people. That function also doesn't apply to anyone who is unable (or unwilling) to have children themselves.
At this point, the functional mindset switches gears - then marriage is about X, or Y. Each of those functions may be needed in society - but there are few marriages that embody all of these functions. There's few that (IMHO) do most of those functions really well. The ability to enumerate the things society does is clearly functionalism's strength - but it's dogmatic lack of flexibility is its weakness.
In a world where things change faster each day, we need the flexibility to make sure the functions of society are met (or improved) without being rigidly held to the dogma that held years ago.
I mentioned this all before when talking about lesbian mothers; the arguments both there and here are principles. There is no such thing as something that people automatically should do.
There are always choices.