In the olden days, it was TRUE and GOOD for a man to support his family: his wife-for-life, and all the children God chose to send him.
But does that really apply to this situation? A boyfriend shacking up with a woman, no doubt on birth control? Presumably they're both enjoying the physical relationship? Why SHOULDN'T she pay exactly 50% of ALL the expenses? She can get just as high-paying a job as he, if anything she has an edge in the job market. She wears pants, has as good/better education than him, and is an Independent Wahmynz.
If he gives her any discount, he's paying for access to... I won't be crude. But you get the idea. She's basically aiming for a certain socially acceptable form of prostitution here. If it were a male roommate, no one would expect this man to give him a discount. But because it's a FEMALE...which superficially looks/acts like his WIFE, people get all old fashioned in their expectations for him.
BUT...why should they? DOES this woman deserve all the traditional care, concern, and privileges reserved for WIVES of old? I don't think so.
P.S. If she were his "girlfriend" of 15 years, basically a common-law wife, having one or more children with him -- my opinion would be completely different. Then it goes back to the natural law obligations for a man to support his wife and family -- even if that man rejects the Church and Her laws concerning marriage.