If a person has no medical condition causing them to obese, but just simply eat too much and do not exercise, is it a sin to be obese? Are we abusing God’s temple that he blessed us with?
If a person deliberately abused his body by eating too much and became obese, yes, it could be a sin. The key word there though is deliberately. Many people who are obese do not become so by deliberate choice, even if they do not have a medical condition that causes it. For some, they may not have been taught healthy eating habits in childhood; for others, they may have grown up around obese people, who were models for them; for others, the extra poundage may have built up so subtly that they did not realize what was happening until it got out of control. In most of these cases, food was as much a crutch for them as alcohol and drugs can become for others; and, for them, it can be a harder addiction to break since they cannot entirely give up eating but must learn to eat the right foods in moderation.
In short, yes, the abuse of food can be sinful, just as it can be sinful to abuse tobacco, alcohol, or drugs (cf. CCC 2290). But just as addiction can mitigate culpability for those who abuse the substances the Catechism of the Catholic Church mentions, so addiction or other circumstances can mitigate culpability for obese people. In charity we should presume that obese people do have such mitigating circumstances and are not deliberately choosing to abuse their bodies.