When it comes to sexual subjects, it can be really tricky. I actually have a warning on my comic saying "there's some violence, there are religious issues, there's nudity, and homosexuality, etc happens in this comic." I don't think these are really things that need warnings, necessarily (and I've had people give me shit over including homosexuality in the warnings list), but I do it so that people who are homophobic or religiously closed-minded, prudes, etc etc will stay the heck away from my comic. It's not for them, obviously, and I'd rather they know that before they get started.
I also have the opposite problem, where I have nudity in the first chapter (not even sexual nudity, really) and I know some people will start reading my comic, realize it's not porn, and then unsub it. And that's okay! I'd rather they unsub then write comments saying "needs more tits". It happens, and it's nothing to get upset about, so I hope the OP doesn't get their feelings too hurt over it.