This is tough. Nearly all of them are sexual, but a relationship depends on both parties agreeing that there is a relationship. What if you sleep with a guy once and you want to start dating, but he thought it was just a fling? What if you just wanted to have cybersex with a guy once and he keeps contacting you for more? Neither of those are relationships if one person has no interest in repeating the experience.
Cybersex can be part of a sexual relationship just like a one night stand can be part of a sexual relationship, but I don't think that doing any of these things once implies a sexual relationship. Both people have to do the act multiple times or intend to do other sexual activities together in the future for there to be a sexual relationship. But if two people meet online and have cybersex once with no intent to repeat the experience, I wouldn't say they're having a sexual relationship.