You have to decide how you want the runs to be and if you want to have formal rules. There are Facebook groups, for example, that specifically forbid members from discussing certain kinds of comments or topics. You could do this and make sexual comments a forbidden topic. You will likely run into difficulties in determining what actually constitutes a sexual comment. Saying someone has a nice ass is pretty clearly sexual unless it's a woman who owns a donkey. Maybe saying she's really hot is also sexual. Maybe not. But what if someone just says a woman is really nice looking? And in the world we've got now it may be just a matter of time before you lose members because someone makes comments about politics or transsexuals or climate change or immigrants or heaven only knows what else.
You're making an assumption here as to why numbers are dropping. There could be things at play that have nothing to do with comments people are making. And if you try to control what people can say you may end up driving more people away. I'm not sure how much I'd enjoy running in a group where I have to think about whether something I want to say is going to get me kicked out.
What used to happen with group runs before such things were formal things is that a few of us would decide to get together for some sort of run. If you liked the people, the routes, the conversation, the time of week, etc., you'd come back. If you didn't like those things you didn't come back after a while. I think you could be setting yourself up for major headaches if you begin trying to regulate what people can say. On the other hand if you're the organizer you may need to toss someone who's consistently obnoxious. So after all that I'd suggest trying to get in touch with people who'd once been coming and now don't and ask why but you are always going to have numbers that wax and wane.