Runbike is right as in this situation it asn't the case that Op didn't go back to work to save money/due to unaffordacle childcare but because she didn't have to - does part time work because she wantsw to, not to be bored.
In the UK (and may other coutries) there is no automatic 50:50 split, depends on the size of assets and circs. And that's reasonable when the assets are very big and earned by one party. Many women stay at home as a mum but very few of those husbands manage to earn huge amounts of money - so it's the husband's achievemwent if he does.
From that point of view, he is OBLIGED to provide for dc and to keep wife's income as it was during marriage (equal to her average expenditure during marriage - spent on herself and also share of the household costs), and may well be required to either buy her and dc a house or leave the existing house to them if he's wealthy.
Prenups are NOT done with the aim to leave a partner penniless - it's to ring-fence the exrta assets that they may clain but don't 'need' - again needs aer based on what the partner is used to during the marriage so very confortab;e in these case.
Consider a scenario - reverse if you like - where a woman is the earner and husband stays with 3 kids (may or may not also have help from au pair / cleaners). Husband has no assets of his own. The woman is a maverick investor or say, an inventor, and build up a lot of assets, say a few millions (that's what OP's partner sounds like) . Then imagine they get married and then he wants to split up afetr several years or when DC grow up because he's met someone else! Or indeed he becomes a bad husband and she wants to leave. Does he deserve 50:50 of her millions? Of course not! The law (and I thikn it's fair) will already give him a very generous allowance based on his expenses during marriage, but an agreement is to prevent him going greedily for millions, way above his needs. I've already seen a very successful woman on MN saying she won't get married fir this reason as she's the wealthy one.
Yes, it's never nice to know that your partner thinks you might leave him or that he may leave, but really it's life and it happens. The fact hat he's marrying is already more than many wealthy people do (look at Mick Jagger, for one, even in his old age!). He knows that by marrying she's going to be finasncially protexcted for life and entitled to his pension unless she re-marries. It's purely to ring fence his future extra millions if things go sour. But yes, after 13yrs it'd be nice if he was a less bothered or cautious.