I think you've put it in the best possible light that you can.
Relationships require maintenance, and keeping in relative shape is part of that - just like dressing up occasionally, sharing feelings, date nights every so often, etc.
I know lots of guys who get in ____ for getting a spare tire or dressing like slobs into the 'comfort' stage of a relationship. It works both ways.
I don't think it has to do with just a bit of weight gain. A lot of people, men and women included, look their best and try hard before getting into a relationship/marriage. It is understandable and reasonable to let oneself go a bit after finding the right partner.
But I have seen more often than not. That people just stop trying. I equate that to laziness or neglect. If they are lazy or neglectful about their body. What else are they being lazy/neglectful about?
Weight gain because of laziness or neglect = ending a relationship for me (again, I am not talking a few pounds here and there). Other medical factors and old age are excusable.
I guess it varies from person to person but my reasons for working out don't really revolve around scoring chicks. Being in better shape, dropping 50lbs so far this year, does make me look better and i suppose it'd help me pick up chicks.. but that's really just a happy side effect.
Ultimately my fitness revolves around competition. The better shape i am in the better i can handle a Superbike, I've also got into duathlons and competitive running and cycling this year. My running still isn't great but I'm pretty strong on a bike, it's just in my nature that if i'm going to do something I'm going to try pretty hard at it. That's just my competitive spirit, it doesn't really have much to do with impressing women.
I have pretty strange standards when it comes to women... I fully accept that that contributes to why I'm single most of the time. The qualities i look for are pretty rare. Not very many women are as athletic, competitive or strong willed as i hope them to be.