In my dealings with the religious, especially those of my family who have remained within, there's been an occasional grace-note of jealousy. They rightly perceive the greater freedom afforded by a life that is no longer guided by arbitrary restrictions created to keep the priestly class in golden vestments and the rest of the people quiet. "You just want to sin" is a formulation that "presumes facts not in evidence", as the lawyers like to say on TV.
First, that word "sin." Doesn't that come up a lot in conversations with the religious? Doesn't that come up pretty much nowhere else at all? I was lucky; the Jewish tradition in which I grew up helpfully divided sin into two major categories. One is between a person and God, and the other is between one person and another. The first category includes what we can safely call all the "victimless crimes" of Jewish law: eating shellfish or cheeseburgers, driving your car on Saturday, wearing a wool/linen blend, and so on. (By the way, wool/linen is the only fabric blend prohibited by the Bible, so the very popular tweaking of religious people about cotton/polyester etc. should stop now. KTHXBAI) Yes, I became an atheist around the same time I shuffled off the coil of these arbitrary and rather silly restrictions, but it's not like I needed to do either of those things in order to do the other. Correlation is not causation.
The second category includes everything that I would classify as genuine morality; everything that Hillel, Jesus, Confucius, and hundreds of others intended when they formulated their versions of the Golden Rule. Becoming an atheist has, if anything, made me even more careful about avoiding this kind of transgression. We only have one life to live, and we only have the other people here with us to live it with. No "world-to-come" promises eternal future reward to justify treating someone in the world right here right now like crap. So if "sin" is a thing at all, it's nothing more or less than treating someone poorly for no reason.
The notion that I became an atheist in order to sin is, on one hand, an insistence that there is indeed a God who indeed cares very much whether the fish I eat has fins and scales or is just delicious without them. That's the silly part. Where the ludicrous becomes insult, shading to injury, is if the speaker is proposing that becoming an atheist somehow freed me to treat people badly.
Because what atheism really clarifies is: people, not gods, are the only fit objects of honor.
This is #7 of a series covering the top ten goofy things religious people say to atheists.