The more she denies it, the more people believe it...
It’s their job!
JC is not some new guy she’s just having around, it’s (literally) his job to be there and her job to spend time with him. We have to stop assuming that two people are dating just because they’re a man and a woman who spend some time together on TV. In all likelihood, we haven’t seen the baby daddy yet either because he doesn’t want to be on the show or Kailyn doesn’t want him to be on the show.
FYI
Believe it or not, the Teen Moms probably interact with tons of people while filming that we don’t get to see, as MTV can’t air footage of someone without their consent. The only reason we see anyone on the show is because they want to be there/be shown on screen (and have signed documents to that effect).
Other options?
For posterity, other names that have been thrown around are Javi Marroquin, Jo Rivera, and Kail’s friend, Tyler Hill. The first two don’t seem plausible either, given that she and Javi are fighting like crazy and Jo is with someone else.
As for Tyler, Kailyn has denied that she and Tyler ever dated.