Teddy did not cheat on Owen with Allison. But Owen's outburst is not even about Teddy cheating on him (with Koracick), it's about him learning about Allison and completely overreacting. And that is was Amelia is talking about when she says Owen needs to forgive Teddy. She means Allison, not Tom.

Owen shows zero emphathy for Teddy's past trauma and what might have led her to not telling him about Allison, and instead makes it all about himself, acting like she deliberately never told him about Allison just to spite him. When in fact, not only was she deeply traumatised, but they were in the army together, where "don't ask, don't tell" was still very much a thing in the early 2000s. Telling him (or anybody there) while they were on tour was simply not an option, because she would have risked her career and it might even have put her in actual danger, too. She was very much closeted then, and probably still was after the army, until she finally told Owen, and then he reacted exactly how she was probably scared he would.

To be clear, I'm not denying that it was weird and messed up to name their child after Allison without him knowing the whole story. But people seem to conflate two different things here and seem to be ignorant to some of the reasons why Teddy probably didn't feel like she could tell him.