I think sexting is perfectly fine. Why would it affect whether or not the relationship is long-term or not? I'll be honest—my girlfriend lets me have sex with anyone I want to, so no one I've had sex with so far have meant anything more to me than a friend, and sometimes not even that. However, not even THAT is damaging to the relationship I'm in.
Also, sex is a common thing in relationships, so how would just sexting have any effect?