This is not a proper foundation to build a marriage on. Dont do it you will regret it.

Totally agree.

There's million of fish in the pond! Find one in the meantime until you get the funds...if you know what I mean. :~)

Don't marry her, if she is doing this to you as a girlfriend I can't imagine what she would you if you ever get divorce

From what I understand, that literally solves all relationship problems. 

If you value your relationship, give in to her demand.

Absolutely.  It was my go-to response for how to get a relationship to implode prior to getting married.

Ditch her, she is manipulating you into making a decision you might not feel ready for buy perhaps were willing to have a proper discussion about... But no, she had to be silly and blackmail you with-holding sex until you put on her a Diamond ring, hahahaha.

Listen to me. I'm 52.years old and happily divorced. DON'T DO IT.
Your girlfriend sounds very manipulative by holding out like she is. If you give in the next thing will be no sex unless you buy her a new car. Then no sex unless you buy her that expensive jewelry. It will never end.
Tell her to stop trying to manipulate you or your getting a new girlfriend.

Ok instead of all these extreme measures.

Have you tried talking to her and saying what you just told us? What is she saying about it?

If she still insists on that blackmail after you said that, then you are gonna have to tell her "so be it". Because there are more important things in life than just sex. Especially in your case cause you got children.