Well, my main concern would be online safety and vulnerability to cyber-predators. There is scary stuff out there, including middle-aged men who can quite handily impersonate randy 14 year old boys or nice 10 year old girls online. I think that your comments about whether or not they would be attracted to each other in person may have confused the issue a bit - it's not just that getting an online boyfriend when you're 14 may lead to disappointment, it's that it is dangerous. Her feelings are totally normal and age-appropriate, but like most 14 year olds, it's hard to really appreciate risk in a concrete way.
It's done now, but I don't know that the grounding was the clearest response. Did she know what she was being grounded in response to? I.e. was it the sexual overtones of their conversation or the safety issue? Perhaps you and she should sit down and work out some solutions together - learn together about online risks, talk about how to have safer relationships, maybe set some groundrules to help you rebuild some trust, e.g. computer filter or password, use during the day only, etc.