Legal protections for me.
I wanted children. I was not prepared to have children without the legal contract that gave me a set of rights that would give me some security since I was going to take the career hit.
I am not romantic. No hearts, flowers and happy ever after for me.
If I'm having your children I want half your pension if you decide to fuck off one day.
Not every woman gives up work and makes the compromises and for them, the legal stuff around pension, housing, etc is therefore not a factor. Arguably they're the most sensible out of the lot of us!
If you are going to choose to have a gap in your NI or take a career break then you get yourself covered legally first imo.
Secondary consideration is the next of kin stuff. Who make the decisions should the worst happen?
Which leads to wills. If you aren't married and your partner dies without a will, guess who is not inheriting. Yup. You.
Basically I am not romantic and I don't trust anyone enough to not ensure they sign on the dotted line before I make myself financially vulnerable as part of a becoming a family unit and assigning responsibilities deal.