I find it odd that weather is being cited as the major inconvenience in this case. People all around the world hold black tie events in all kinds of weather. But black tie isn’t just a dress code – it’s a level of formality for the event you are throwing (evening affair, valet service, butlered hors d’oeurves, formal multi-course dinner, multi-piece band and so on and so forth).
So if you are throwing a gala event that lives up to the expectation of black tie, then the corresponding dress code for that really shouldn’t come as surprise to your guests. Additionally, you as the host should be hosting it in an appropriate setting where they wouldn’t feel uncomfortable in that clothing, i.e. an air conditioned or appropriate climate controlled ballroom rather than outside if outside would be too uncomfortable for that type of event and the clothing that is appropriate for it.
I would be far more concerned about the convenience of bringing tuxes and gowns to a destination wedding than I would be the outside temperature (although I guess temp may play a role if you’re throwing a more casual outdoor affair, but then tuxes and gowns wouldn’t be appropriate for that kind of event anyway). And also the costs associated with being a guest at a black tie event above and beyond their travel costs, which quite frankly is already a big ask in most circumstances.
The questions I would be asking instead of dressing up when it’s 80 degrees:
Are my guests familiar with black tie events? Do they routinely attend them? Will my guests potentially feel overwhelmed or out of place with that level of formality? (I know we all would like to think that being able to experience black tie will make people feel special, but there’s also a good chance it might make them feel awkward or out of place, depending on the types of events are usual in their social circle).
Do they own tuxes and gowns already? If so, does the addition of having to bring these items mean potentially checked bags, or needing ways to launder/steam/press once they get to the destination, or other inconveniences?
If they don’t own tuxes and gowns already, is having to rent and or purchase tuxes, gowns, and the appropriate shoes and accessories convenient and affordable for them? Keeping in mind that this is on top of their travel and accommodation costs?!? Is there a place to rent tuxes at the destination or will they need to pay additional fees if your destination wedding means they’ll be gone longer than the traditional rental window?
Personally, I think weather/temperature, is the least of your concerns when making this decision.