Itinerary advice needed for trip to Italy and Spain (Honeymoon)

Consider Flying into Venice and out of Madrid or Barcelona.
Once you get the airports you will fly in and out of then your itinerary will take shape in order.

I've heard you only need a day in Venice.

That is not a standard. Everyone is different.

You might consider Venice to Florence to Rome to Amalfi, since you say no more than 3 weeks.
Then fly to Madrid or Barcelona from Rome or Naples.
In Spain maybe include a trip to Segovia, Toledo, Seville, Granada

Just some planning considerations. August is a beastly hot month in Italy and Spain. Beach space is at a premium, and the cities empty, leaving many businesses in Rome and other inland cities shuttered. Madrid also becomes more like a ghost town.

Venice deserves at least 3 nights. Together with Paris, it is one of the world's most romantic cities.

The Amalfi coast is not famous for wonderful beaches. Mostly pebbles and generally smaller and (see above) very crowded.

Forget Madrid futbol. Barcelona rules.

You need to decide what your goals are, so to that end, I suggest that you purchase a RS guide to Italy (2018 Edition) and the RS guide to Spain (2018 Edition) and do some reading on the two countries. in addition, you can stream his videos from this site which will give you an idea on what you might want to do. Right now you don't really have an idea for a honeymoon trip--you have an idea for an idea for a honeymoon trip.

I think three weeks for two countries is doable, but rushed.

Day
1. Arrive Venice (3 nights)
4. Train (via stop in Florence) to Rome (4 nights)
8. Train via Naples to Amalfi Coast (5 nights)
13. Fly from Naples to Barcelona (4 nights)
17. Train to Madrid (4 nights)
21. Depart Madrid

You need to check historical weather statistics so you know what you may be experiencing. Very high-level summary data (monthly averages) can be found on the Wikipedia pages of major cities. For more details (to see how hot it actually got, day-by-day, during the Augusts of recent years), go to wunderground.com. Choose "History", then "Monthly". Perhaps you'd like to spend some romantic time up in the Dolomites in Italy. The Basque Country is Spain would also be a fine idea.

One night in Venice--and your first night at that, when you may well be too sleep-deprived and jetlagged to function--is just plain silly. I'll bet the people who said that only spent a day there themselves, and they're setting you up to dislike Venice as they apparently did. Either give yourself enough time to get into the back canals and appreciate the city, or skip it entirely. Italy has lots of great places; there's no rule that says you have to go to Venice, especially when you only about ten days for the entire country.

It's really not a good idea to go to Europe if you want to spend a lot of time on beaches. The accessible ones are much, much more crowded than the typical US beach. Plus, many of them are rocky rather than sandy. If you foresee a romantic beach vacation in southern Europe, you may be very disappointed. You'll be sharing the space with what seems like half the population of the United Kingdom, Germany and Scandinavia, plus all the locals who take their vacations in August (which is a very large percentage of them.)

Everything is relative to what you are used to.
I don’t consider these August averages to be particularly ‘beastly’.

Venice 18c - 27c
Rome 18c - 30c
Sorrento 18c - 29c
Barcelona 21c - 29c
Madrid 16c - 33c

I don't know where Jessica is from or what she's used to, but I just checked Wunderground for Madrid, and last August there were 21 days when the high temperature hit 90F or higher. Sixteen of those days were 95F or higher. That's why I don't like to rely totally on averages, some of which apparently are based on figures going back a very, very long time, pre-climate change.

Similarly you can’t rely on the occurrence of a heatwave period last year to be a predicter of a similar event this year.
I spent 2 weeks in Madrid one August. The days are hot but dry. The evenings are, by contrast, really very pleasant.
nb. It’s currently 96F here in Brisbane as I write this.

I agree with the previous comments that you should allow 3 nights in Venice. I think you will regret not staying longer in Venice. I don’t know anyone on this Forum that would suggest you leave Venice on the next day after you land. You will still be recovering from jet lag. Venice is unlike any other city in the world. It would be better to just fly directly to Rome than staying such a short time in Venice. It reminds me of a visitor asking a Park Ranger: “ I only have one day to see Yosemite, what should I do?” The Ranger replied: “Cry!”

No, one year doesn't predict the next. However:

August 2016 days when Madrid hit 90F or higher: 29
August 2016 days when Madrid hit 95F or higher: 14

August 2015 days when Madrid hit 90F or higher: 22
August 2015 days when Madrid hit 95F or higher: 10

I live in a city where hot and muggy is pretty much standard in July and August. It's not uncommon for us to have several near-90F days in a row in April. But as we've discussed on the forum many times, it's one thing to deal with super-hot weather when you have an air-conditioned home, workplace, car--really, just about every indoor place you encounter; it's a different matter when you're traveling and out in the heat much of the day and cannot count on finding an air-conditioning when you step inside a café, shop or museum.

Honestly, every time I go looking for actual summertime day-to-day temperature stats on wunderground, they seem markedly worse than one might expect, based on the monthly averages on Wikipedia. I usually don't pay any attention to the date-range for Wikipedia's averages, but sometimes I notice it, and it always seems to cut off at something like the year 2000.