Why?
Other than an excuse to have a party, I don’t see a reason.
Well, it is after all an excuse to have a party-- That’s one reason right there. It simplifies the question of “what do we call each other” considerably. It might save you money on your taxes. It helps you cut through a lot of bureaucracy. It should be well-known by now that marriage has literally thousands of legal benefits, and those all apply to straight people just as much as to gay.
Or that they are running a business together.
I like boyfriend and girlfriend and I’m over 40.
I have a friend who refers to her guy as The Manpiece.
The meaning of a word is not derived solely from that of its constituent morphemes.
The issue is not that the word has “girl” or “boy” in it, it’s that the whole concept of girlfriend or boyfriend sounds, to some people, like a temporary state.
I prefer to use, “emergency contact”.
Just call them your lover. Especially if your adult children are around. That’ll go over well. 
We have a private joke we use and so far no one has asked what it means. She is my sop and I am her sod. To most of the neighbors and acquaintances we are husband and wife.
Source of pussy and source of dick.
See, I’ve only read this (in fantasy/historical novels) as a polite term for “whore” or at best “mistress” - the lower-class woman that a noble fellow is screwing regularly, but wouldn’t ever consider marrying, though he might or might not acknowledge any resulting bastards.
Not that there’s anything wrong with that.
Actually, I have heard straight SO’s referred to in that way probably as often as gay ones.
Before we were married, I used “partner” and sometimes “SO” if I wanted to be more vague about the intimacy of our relationship. In my will at the time I used the term “life partner” but that was too formal for everyday speech.
The lady I was seeing really hated “girlfriend”. I researched the matter online, and “leading lady” seemed nice to me, but she hated that too. Now she has decided that “former lover” is the appropriate title. Kinda sad about that, but as breakups go, this one is great.
I know an old biker dude who still refers to his gf as his bitch. It makes me cringe, but she’s fine with it and he insists it is a term of endearment.
Almost as well as “common law bedmate”. 
I’m speaking ex cathedra from the seat of my pants, here, but I seem to remember “significant other” being coined as a parody example of political-correctness-gone-mad (along the lines of “follicly challenged” for “bald” and “Gyno-American” for “woman”), and then gradually being used in a non-ironic sense.
What about “companion”? I’m very fond of that, because it literally means “the one you break bread with”, which has a nice homey feel to it. I don’t know your father-in-law and his friend, but I’ll bet companionship is primarily what they provide each other. And it’s a wonderful thing, too.
We say boyfriend/girlfriend. He doesn’t mind, but I’m not wild about it. If the context works I call him Sweetie.
My 76-year-old mother insists on introducing him as “Helena’s Special Friend”. :smack: :rolleyes:
Boyfriend always felt strange to me, so I’d use man-friend.
Wife. My country treated us as married. My religion considered us married. My friends and workmates treated us as married. Marriage is something you do, not something that happens to you. Husband is something I do, not something I did.
I am single now, but “parasitic mate” summed things up nicely with regard to a past relationship.