As long as your partners are over 16 (and you are too), there's no exchange of sex for money, and the sex does not take place in public, meeting sex partners online is as legal as meeting them in a bar, grocery store, church, dentist office, etc. Good luck!
To reiterate the sentiments of my good friend Mr. Wagnild, there is nothing illegal about that so long as the ages are correct. The morality and/or safety of it is for another discussion, but so long as no money is exchanged, and they're all consenting adults, you are probably good to go.
If you're not paying for the sex act itself, and the people are old enough to consent, you're fine. I can't reiterate enough- make sure they're not minors! The site usually has a policy to not allow minors to sign up. But, as an added level of protection to yourself, I would make sure they're legal if they look young. I had a client who got into similar trouble, and it was a mess to sort out.