Is it “acceptable” for a male wedding guest (not in the wedding party) to wear a tux?
Someone asked me if he should rent a tux for our wedding (he didn’t own a suit and figured it was cheaper to rent a tux than to buy a suit he’d never wear). I told him he was free to do whatever he wanted, but he’d be more dressed up than the groom.
I don’t see a problem with it. The only form of dress that should be avoided is for women: they shouldn’t wear something that looks like a wedding dress, and if they know what colour the bride’s attendants are wearing, they should avoid that too.
As Heart said … if it’s black tie, yes. Otherwise, you’ll either be overdressed or underdressed (in the extremely unlikely event it’s white tie.)
FWIW, I can’t think of any wedding I’ve ever been to that was black tie. Even Prince William’s wedding wasn’t black tie.
He’ll look silly. It’s not a hanging offense, but it would definitely be a faux pas.
What’s the dress code on the invitation? I’ve been to plenty of ‘black tie’ weddings.
Look at the invitation. If it says “Black Tie”, and you’re a guy, wear a tux.
If it says “black tie invited” or something similar, you can pick either a tux or a dark suit, and neither is “inappropriate”.
If in doubt, why not make a quick phone call?
Point of note: a “black tie” evening dress (tuxedo in American parlance, “dinner jacket” in Europe) is supposed to be worn at evening functions, i.e. those that occur at or after dinner. Formal dress events during the daytime require morning dress. There is no event that requires a velvet jacket or brightly colored braces and sash unless you are a clown or washed up rock star.
As a practical matter, a dark two piece suit with a somber tie should be acceptable for nearly any formal event that is not specifically black/white tie or a state dinner.
Stranger
That’s because it took place at 11 o-clock in the morning; black tie should never be worn before sunset or 6pm (whichever is ealier). Morning dress is the approriate dress code for a formal daytime wedding. The private dinner that evening was black tie. White tie is pretty much onheard of for weddings. Even royalty doesn’t do it.
Back to the OP; it depends on how formal the wedding is and what time of day it’s taking place. The wedding invitation should be you’re first clue. Does it actually say black tie (optional)? How is it worded (eg “Mr & Mrs John Doe request the honor of your company at the wedding of their daughter Mary to Mr Richard Roe…” vs “Mary Doe & Richie Roe invite to their wedding and garden reception”)? Is it a plain white card with black lettering? Is the wedding/reception outside or inside?
I think I just found the answer. The wedding invitation doesn’t mention dress, but the reception (“to follow,” in another part of the city) is “black tie preferred.” The wedding is at 5:30 . . . so I’m assuming people will not run home and change, but wear their “black tie” to the wedding as well.
It’s acceptable (well expected) for male guests to wear black tie then.
Does this still count if the bride maids are wearing black?
I attended a wedding where the bride’s mother wore a black dress and the bride’s sister wore a white dress. I said nothing, but I did ask myself, “What were they thinking?” 
My advice to the OP: call and ask. Regards,