How come I see very fit men with really fat girlfriends?

I mean I would say if I was a fit man, I would want something that is just as fit as me! But alot of men I see they stay at the gym and get toned up for a woman, but the woman they get toned up for be fat as hell! Now I understand if you trying to get the woman in shape, but alot of them date unfit women and don't even hestiate to get them in shape! I mean its weird as hell and doesn't make any sense! You in the gym trying to look good for your mate but yet your mate aint trying to look good for you! That's backwards really! But how come that is tho?