It's manga. "Male MC accidentally barging in on naked girl in the bath." is required by manga law. Just be glad "and both accidentally slipping so that male MC lands with his face buried in her breasts." is somewhat optional.
To be fair that is a really nice set of armor, he was too focused on the craftsmanship of it, also he didn’t know she was naked since there wasn’t any clothes in there aside from the armor. He had yet to realize that they don’t wear clothes and only wear armor, by the time he did it was already too late. Glad she didn’t over react or anything, it was just her lizard which attacked someone it thought was sneaking up from behind.