I think it’s important to give context on my own history.

I dated my now wife for over 5 years before we got married (didn’t want to get married prior to graduating college). By the time we were in college we freely spent money on eachother, and had no problem giving eachother some money as long as it didn’t break the other person’s finances.

In your scenario had I won a fair amount of money, I would’ve gladly given my then girlfriend half, that said there is 0% chance she would’ve made any demands, and knowing her she would’ve very likely told me she wouldn’t accept half.

Now as a married couple of over 10 years with joint finances, we don’t see ‘my’ money, ‘his’ or ‘her’ money, rather it’s all ours regardless who earned it; we even have equal allowances we both equally agreed upon. The one exception is inerhitances and/or gifts given to a specific person for a specific reason. With inheritances with no strings attached (like nothing in the will saying “use this for your kids college”) in that case we do actually say “hey, it’s yours first before it’s ours” and whoever had the money left to them will normally use ¼ or 1/3 the money on something they’ve been saving for before the rest hits the budget. For gifts given for specific reasons we use that money exactly as intended, like “hey I know you need to get your house painted, please take this money for that” and we spend it on fixing up the house, not going on vacation and then buying some crappy paint and paint one room.

I guess what I’m getting at is however couples decide to spend/share money, it should be a meeting of equals and decisions should be made out of respect, not demands. I think the fact he’s demanding from you is pretty sad.