How can we avoid big money in casual games?
For my suggestion of a house rule to use, please see the section of this answer titled "Embargo Counters".

If I understand correctly, you were learning the game naturally by playing it - it's just that at the current stage of learning it you have got to, it isn't very interesting, which makes it harder than it has been to carry on learning it simply by playing it.

So, it may be that all you need is to see, or be shown the strategies that are more powerful than Big Money in a natural way, so that you can have fun exploring them, rather than feeling forced into big money all the time.

As part of that I would draw your attention to this list; http://councilroom.com/openings. This is a compiled list comparing how good the first two opening moves are. Silver / Silver (so buying silver with your first and second purchase) is only rated as a +1 - there are 962 better opening moves. The best move involving silver at all is rated as a +4, with 86 possible openings rated as being better.

Similarly the Thief isn't especially powerful, and wont show up highly on the list above, but does actively and obviously combat BM. With no moat or other defence cards in play BM could look a lot less attractive with a Thief available, particularly if there are some action cards that give money, like festival and market out as an alternative.

My suggestions

You could intentionally play a few games with cards specifically from the top of this list (same as linked above), and set a house rule like those suggested in Jefromi's answer, my suggestion involving embargo counters at the end of this answer,  or those in other answers, to force you to experiment with these strategies, while still enjoying the game. You should find yourself exploring ways that beat BM anyway. I suggest you employ the lightest touch home rule you can find that does the job in pushing you both away from BM.

You shouldn't need to always play with any particular cards, or with house rules, but seeing and playing a deck built around the cards at the top of this list should help show most obviously and quickly that BM isn't the best strategy. And hopefully do so in an enjoyable way.

Play with others

Having a few games playing with other people - who know the game better than you do. You will hopefully quickly see a wider variety of strategies, without having to put effort into learning them. It might help to see a strategy like Chapel being played for example - it is one of the most powerful cards in the game, as you can see from it's presence in a fair number of the best openings in the list linked above, though you never want more than one, and it's power is not immediately obvious.

Prosperity

Lastly, the top answer here: How can I beat &quot;Big Money&quot; in Dominion? suggests counter intuitively that including cards from Prosperity advantages Kingdom Card strategies, because it slows the game down. - I realise that question isn't the question you are asking, but thought it'd be worth pointing out as a potentially subtle way to further disadvantage big money. That said, it probably wouldn't be enough on its own.

Embargo Counters

It might work to put a small number of embargo counters on each coin denomination at the beginning of the game, so that whenever a player buys one, they gain that many curses. This house rule has the advantages that it should make BM less attractive while not preventing certain other kingdom cards from working, it uses a game mechanic and it doesn't require constantly remembering that you aren't using the cards as printed.

The disadvantage of this home rule is that it could be too heavy handed, particularly at the start of the game (especially with certain setups; for example one that manages to combine no ability to trash curses with no alternative, low cost sources of purchasing power).