Well first off that's a stupid argument.
agreedagreed when it comes to harming others, it should always be against the law to harm people that don't want to be harmed* and it's a bad argument to say "well, a small fraction of men are going to rape, what's the point of making it illegal?"....but is it that bad when it's about self harm? People are going to get high, sometimes in ways "normal society" doesn't approve of. I think it's wrong to punish those people for choosing to alter their brain in ways your aunt Ruth doesn't approve of. People were going to have gay sex 50 years ago, risking everything to do it. People should be free to do whatever they want to do (as long as it't not harming anybody else). That includes DMT, butt sex, bacon and suicide (now that's a fun, final weekend! Better than rotting away in an old folks home surrounded by smelly old people, ignored by your children.).
*but then how far do we take that? Parents of fat kids (with the obvious caveat that excludes the TINY percentage of fat people that are fat for medical reasons) are certainly harming them, do we punish them? how?
Normally when arguing this with a social liberal, I'd just point out the obvious hypocrisy of opposing social conservatism on those grounds but then supporting seatbelt laws and a host of economic regulations. However you are a consistent libertarian so we can get to the meat of the issue:
There is a tendency in liberalism (both economic and social) to atmomize people in a way that doesn't really reflect how people actually live. I dispute a lot of liberal claims of "they aren't hurting anyone but themselves". The distinction between harming only oneself and harming others is rather artificial when applied to real life situations.
Examples of this include divorce and drug abuse. Two parents can consent to a divorce but the act can still dramatically impact their children. Likewise I have yet to hear of a heroin user who didn't harm others in an attempt to feed their habit.
So if actions that only hurt oneself are a lot rarer than what libertarians make it out to be, should the state criminalize every vice? No. The state should weigh a variety of factors when considering banning or regulating vice including: how much it harms the user, how much it harms others, how endemic the vice is to a culture, how much regulation would impose on citizens etc etc.
To use your example, making kids fat is bad, but the harm is relatively small, the infringement on parental rights is large and the potential for a government abusing their powers to take kids away is excessive, so I wouldn't favour regulating children's weight.