How should I decline when someone asks to exchange LINE at the first meeting?

If you’re using your real name on LINE, you should change it if you’re going to engage in this kind of questionable activity. Otherwise, you’re basically letting your personal information spill everywhere. As long as you don’t use your real name on LINE, there’s no real problem with exchanging contacts—you can just block them right away afterward.

If that’s how you see it, you’d be better off going to a sex service rather than a dating club.

Is it because you don’t want them to know your real name? If so, you could simply change your name on LINE. And if you don’t want to do that, you could suggest using a different way to stay in touch. You’re an adult, so surely you can come up with your own reason for suggesting an alternative to LINE. Asking other people how to lie—that’s really something. Personally, I find people who start off with such pointless, obviously made-up excuses too much trouble, so I wouldn’t continue dealing with them. Adding this after reading Kamihikōki’s answer: I’ve used adult entertainment services twice in the past two years, and both times, the woman I saw asked to exchange LINE contacts. I assume it was to encourage repeat bookings, but apparently even women working in that industry exchange LINE contacts with customers these days.