I've been dating this girl for about eight months now. We communicate well and know when to argue and when to just let it go. Recently she put on weight and I'm starting to find her less sexy. (I'm not a fitness nut but like being healthy.) How do I approach this subject without hurting her?
I don't think you're at the point yet where you need to address it through a conversation. For one thing, most people's weight fluctuates a little and there's a good chance it'll drop back down in due time. Moreover, a conversation like that is risky even if done with genuine care. Instead, I'd suggest you start by making a couple modifications to your routine.
The first thing you should do is make sure you eat healthy. You don't have to make it obvious that you're eating healthy..."YES, HI, I'LL HAVE THE LIGHT SALAD WITH THE LIGHT LETTUCE AND INSTEAD OF DRESSING I'LL JUST POUR MINERAL WATER OVER EVERYTHING. What kind of hamburger did you order again?" Just try and make conscious choices about what you're putting in your body.
The other thing you can do is exercise together, but make it fun. You can run or jog together around your neighborhood and set up occasional race, winner gets oral. (Note: make sure you will win, and if it looks like you're going to lose, cheat.) Another option which seems to be gaining popularity these days is hiking. Hiking is nice because you can set up a picnic with a bunch of overpriced organic food when you get to a clearing and do the nutrition thing as well. I personally find the local hikes to be incredibly boring and unchallenging but if you're with your girlfriend, maybe you can make it interesting somehow. Somehow. ê
Personally, I'd probably opt for lifting weights together. The payoff is pretty immediate (feeling calm afterwards and sore the next day), and the venue is ripe for joking and fun. I was recently dating a girl who was trying to lose some weight. We'd meet at the gym and I'd force her to wear ridiculous power-bandanas with me. When she'd lift weights she'd complain that the weight was too heavy, and I'd complain that she was a complainer while comparing her strength to that of an infant parakeet. We'd also gossip about the weird habits of everyone else at the gym and their gimmicks for generating attention. It was cool and became a good platform for flirting.
Try and be discreet about it. If she doesn't want to go with you, just nag a little. You can tell her in a very serious tone that you'll be her best friend if she goes with you and stretch out your hand to confirm the deal with a formal handshake. If she still refuses, tell her she's a dick and try again later. As soon as you start noticing some positive changes, give her a sleazy compliment. If none of this works, write back and we'll talk about ways to bring this up in a conversation.