My theory? When you're not in a relationship you need to be at your "fighting weight." If you're out competing (and it is a competition.. especially for females) in the single market you need to be in your best shape.
Once you tag and bag a decent one, you go through that "I want to spend all my time with you" phase... that usually means sleeping in and cuddling and [}:D] instead of hitting the gym. Then you get out of the routine. Your body can stay in semi-decent shape for a few months... usually long enough for the "I love you" declaration which is implied permission to let yourself go. After all, this other person loves you for WHO you are not how you look, right?
I'm not going to lie... I've gotten out of my routine in the beginning of every relationship I've been in. But some folks, like myself, will get grumpy if they are away from the gym too long. It's a matter of priorities... I consider working out as making my health a priority.
FWIW, I think when you love someone, appearance isn't as much of an issue. Then again, I've never dated guys who were overweight and my lifestyle is active enough that someone who wanted to be a sedentary couch potato wouldn't last long in my life. ::shrug::