Fashion has very little to do with attracting partners and a lot to do with impressing your peers. Women try to be fashionable for their friends and coworkers, not their boyfriends or husbands. When a girl dresses fashionably in a social setting with her boyfriend, she isn't trying to keep his attention, she's trying to signal "You can't compete with me" to other women* (this will end the instant they have children). Men are much more likely to dress 'schlubby' when they have a partner because they don't depend on their looks to stay in competition with other men. Stay solvent, brush your teeth and remember your anniversaries and you can wear and drive whatever you want.

*This is frequently seen in popular culture in the form of "low-status girl is afraid high-status woman will steal her (percieved) high-status boyfriend, even though he loves her and sees through high-status woman's play." There's probably a TVTropes article on it. This only works because the low-status girl is usually as attractive as the high, just not as well-dressed or made-up; in reality, men with less attractive girlfriends often cheat with more attractive women, given the opportunity.