This is a hard question to answer without the context of age, duration of relationship, and knowing a few things about what's been going on in your lives. There is one thing I can say, though, having been on the other side of this - having lost 80 pounds in the year 2000 after some years of being obese, and then being healthy, fit, and having kept the weight off ever since: Nothing that you or anyone else says or does will make your partner lose the weight until he or she is ready to do it for themselves. If you push it, you're likely to get a grudging and short-term effort to change, and there's a good chance they'll resent you for it. They aren't clueless about their obesity and its ramifications, and if they aren't doing anything about it, it's because they haven't made up their mind to, for whatever their reasons.
You can tell the person in words of your choice that you're concerned about their health, and that while you don't love them any less, their obesity is affecting your desire for them, and that that saddens you, because you miss that part of the relationship. Depending on how the conversation goes, you can also add that it concerns and saddens you that they don't care more about their own health and appearance. Said in the right words and tone, in a non-threatening way, it's your best hope of planting a seed and maybe getting a dialogue going. If nothing happens, depending on how important this is to you, you may have some tough choices to make. Good luck. I know more than a few couples who have struggled with this.