Why would white opening with e3 and then f4 be a bad opening?

All that opening with 1. e3 2. f4 does is oprns up a diagonal to your king h4-e1 and I;m sure Black will get his Q to h4 asap with another piece to attack g3 but Roper300's structures look pretty solid so what do I know ?

1.e3 throws away white's first move advantage by not staking a claim in the center. Black is already equal after 1...e5, d5 and possibly c5 and Nf6.

2. f4- f4, in general, is inferior to e4 or d4 because those moves both occupy and attack and central squares where f4 only attacks a central square. Furthermore, f4 weakens the kingside. That difference alone is the difference between the Sicilian/English (which are strong mainstream openings) and the mirror image Bird's/Dutch (which are not).

Lastly, you could reach the same positions by playing 1. f4 first (black can't really prevent e3) but playing f4 first gives white more flexibility because white doesn't have to follow up with e3. There's really no point to playing e3 first unless you're looking to play a reversed French. (which I would consider a questionable idea anyway)

I'll also add that Qh4+ is a huge problem. It has potential to lead to a very sharp position where you have to play perfectly just to survive. But, even if you do, after g3, you're left with weak light squares all over the place and a dark square bishop that's several moves away from being able to do anything useful. Normally, in Bird's/Dutch lines you follow up with Nf3/Nf6 to avoid the weakness on that diagonal and in most fianchetto lines you try to avoid locking your pawns on one color because of the weak squares created. Your line creates both tactical and positional weaknesses.

Thanks, all. I understand now. Reckon I'll experiment with the Bird for a bit because it seems like a fun departure from what I usually do.

And note that Larsen used it to beat the best players of his time. At least 2 world champions and alot of top GMs were the victims of this "dubious" opening.

GM Henrik Danielsen used it regularly and he even wrote a book about Polar Bear system which is Bird with k-side fiancheto(reversed Leningrad Dutch)

Carlsen used Polar Bear to beat Kramnik.

Overall , it is an opening that allows many different systems. There is nothing wrong with it except for the fact that it doesn't create any immediate pressure and allows Black easy equality(but so does Reti yet it is very popular in top level). That and the fact that 1.f4 move is too commital(Reti is certainly more flexible) makes it undesirable in top level. So when you play in Candidates tournament I would recommend to choose something else. Other than that, it's a perfectly fine opening.

To be fair, I think you should post the opposing position also. Show games where white get mated in three moves.

You have to be an idiot or a total beginner to get mated in 3 moves.
I don't need to post defeats for the simple reason that I never claimed the opening wins. All reasonable openings, even some not so reasonable like Bird(1.f4) and Sokolsky (1.b4), are as good as the player's understanding and familiarity with them.My goal was to show that 1.f4 allows a player that is creative to show his talent (assuming he has any). With that opening world champions have been defeated numerous times and top players have created masterpieces , so it is more than fine. I explained the drawbacks and why it is undesirable for those who compete in high level but all these are irrelevant for on line chess.