I think women do go for the guy for stability, but the fact is do you want to be with a girl who is with you for stability while she's shagging her personal trainer behind your back?

I think you should keep yourself in good shape all the time for these reasons:

1: as stated above less likely to be essentially using you as a sugar daddy.
2: you're less likely to get mugged if you're hench
3: you feel pride in your body.
4: you'll have more energy
5: it improves your sexual pleasure
6: makes your cock look bigger
7: you can actually see your cock