Below is an example I posted on the other thread of the sort of issues that can arise. Obviously I have no idea of your personal circumstances op and it's entirely possible you're completely covered on all fronts so not trying to suggest you haven't thought things through! It's just a real big bear of mine that there is so little out there about this sort of stuff!
As an example. Unmarried couple, 2 DC 5 and 2. Both own house 50:50. House is worth 200k with 50k of equity. Dad works full time, earns 50k. Puts a bit into his pension. Mum dropped to 3 days after kids were born earns 20k.
They decide to split up. Sell the house, each get 25k minus fees so say 22k each.
Dad will see kids one evening a week and every other weekend. Will pay maintenance at cms rate so approx £500 per month.
Dad takes home approx 3k per month, mum 1.5k (plus maintenance of £500).
Dad is entitled to a mortgage of approx £200k, uses his share of equity to buy a house. Mum entitled to mortgage of £80k so will find it difficult to buy. Rents a house. Mum has to pay for childcare for her working days. Dad under no obligation to contribute to this as paying maintenance. If mum goes up to full time, childcare costs increase.
End result is dad in a new house, paying a mortgage, paying into pension, seeing kids. Mum is renting, will struggle to pay into pension, is materially worse off every month due to earning less and childcare costs. If mum didn't work at all the situation would be even worse.
If married it is likely mum would retain most or all of the equity in the family home. Mum might be entitled to spousal maintenance as well as child maintenance. Mum may be able to stay in family home for 15 or so years, avoiding the need to rent during this time. When house is finally sold equity will be greatly increased and mum will have had an opportunity to build her career back up as children got older and childcare costs decrease. These benefits are likely to outweigh the legal costs of a divorce very quickly.
I appreciate this is all hypothetical and there will be circumstances that are different. I also repeat that I AM NOT saying people should get married only that they should fully understand the benefits and make an informed choice.