How do Singles respond to, "You have no life", etc.

Never in 46 years been told this... for being single. In fact most of the things singles claim others do or say to them I've never experienced. Nor to anyone I've known.
Really no one ever even brings up that I'm single unless they've heard I was dating someone.
Although I imagine it's not something they'd want to bring up again if they did so. If someone became pushy or critical of my being single they'd be quickly cut off.

Say nothing, let them assume that is the case.

How do you respond to such comment?
It's simple - you don't respond.

You are full of life (so they are dead wrong) and even if you think you have no life, don't give your time of the day to such negativity.
And last, it's time to cut that fellow Christians out of your life if they are around.

I just do what my kid does. "Okay" and then walk away and get on with my life.

They look like they're a happily married couple. I sort of like routine too.

I have really never been told this being single. I have however been told I really needed to get out more when I was married. My ex was a home body. Once a month I do something a little different even if it is just going fishing.

I am single and so far, I haven't been told that I have no life. (Knock on wood!) But if someone did make such a comment, I don't know that I would necessarily respond. I'm not obligated to care what they think, and I like the life I'm living. As long as you're living a life you like, well...you've got one